Love plays a much bigger role in Voldemort's downfall than he or Harry realize. Regardless of Merope Gaunt's intentions for her son, Tom Marvolo Riddle grew up an anonymous child in an orphanage, without the unconditional love or care he would have received from his mother. Whether or not this lack of love in his life shaped him at an early age, it is certainly true that, by the time Dumbledore met him at eleven, he was already showing sociopathic qualities.
Riddle's inability to comprehend love, or the purity of it, kept him from making any real friends. When he meets his father and grandparents at 16, he does not attempt a relationship or any sort of reconciliation. Instead, he murders them all and uses their deaths to create a Horcrux.
Perhaps it is the lack of love in his early life, coupled with the missing pieces of his soul relegated to Horcruxes, that further renders him incapable of understanding love's impact. When Snape approached Voldemort and asked him to spare Lily Potter, Voldemort did make an attempt to do so. Harry's limited recollections of the night his parents were killed, and the memories of that night that Harry later receives from Voldemort, agree that Voldemort had commanded Lily to stand aside so that he could gain access to Harry. However, spurred on by her husband's death as well as her maternal instincts for her child, Lily refused, and Voldemort killed her before turning his attention to Harry.
It is Voldemort's intent to kill Lily if she gets between him and Harry, that turns Snape against Voldemort, and leads to his defection to Dumbledore and the Order of the Phoenix. Learning of Voldemort's plan to kill Harry, with the associated grave risk to Lily, Snape pleads with Dumbledore to protect her, and promises "anything" if Dumbledore will do so. Dumbledore requires Snape to turn informant against Voldemort. Lily's subsequent death at Voldemort's hands does not end the promise, and Snape remains an ally of Dumbledore's until his death. When Harry comes to school at Hogwarts, Dumbledore adds covertly ensuring Harry's safety to Snape's mission. Despite Snape's overt animosity toward Harry, probably in part caused by Harry's extremely close resemblance to James, the older man never forgets whose child Harry is, and feels duty-bound to watch over Lily's son because he loved her so much. Snape's work as a double agent allows him to pass along information about the Death Eaters to the Order of the Phoenix and thwart certain parts of Voldemort's plans. If Lily Potter hadn't been killed, Voldemort would still have had a loyal servant in Snape and Harry might have been successfully killed long ago.
It should also be noted that throughout the seventh book, Harry receives valuable information about Voldemort's actions and plans by means of the soul shard of Voldemort's that is resident in Harry. Voldemort is unable to reverse this flow of information, because of the extreme distaste he has formulated for the inside of Harry's mind. What drove Voldemort permanently out of Harry's mind was Harry's longing to be with Sirius Black again. Dumbledore had commented at the time that he believed Voldemort had found Harry's mind a very uncomfortable place to be, and that he was not surprised that Harry had had no intimations of Voldemort's presence in his mind throughout the sixth book.
The fact that Voldemort can’t feel or understand love, but that Harry can, is also very important. It is because Harry can feel love that he can reciprocate, and it is this reciprocation of love that allows Harry to be surrounded by those who will assist him in his ventures, most notably Ron, Hermione, and Ginny. Voldemort can’t feel or understand love, so those who choose to follow him do so purely for their own selfish motives, coupled perhaps with fear of what Voldemort would do to them if they went against his will. It is this same lack of understanding that blinds Voldemort to the possibility that by failing to spare Lily, he could lose his follower Snape, and this lack of ability to reciprocate love that frees Snape to act against Voldemort covertly. When Voldemort has proven by his actions that he does not care about Snape's feelings, Snape in turn feels that he is free to seek whatever help he can find to protect Lily, and the help he finds is Dumbledore. On the other hand, while Ron is unhappy enough with Harry's progress to leave him, we never think that he would be able to betray Harry. We have commented elsewhere that Harry has friends while Voldemort has followers; Harry's friends stay with him because they believe in him and his mission and want to help him, and because of this, their motivation is much stronger than those who follow Voldemort.
This is not to say that Voldemort does not know how to use love as a weapon. In one instance we are aware of, Voldemort's Death Eaters kidnap Luna Lovegood in order to pressure her father into changing the editorial tone of his newspaper, and incidentally almost manage to capture Harry by that means. Being unable to see the strength that derives from love, Voldemort likely only sees this vulnerability, and thus believes himself stronger for having nobody that he cares about.
Death has played a central role in each book, and is at the heart of the final confrontation between Harry and Voldemort. Both Horcruxes, which form the main thrust of the entire series, and the Deathly Hallows, which are a major part of the story of the seventh book, are shown to be means of avoiding or mastering death.
Practically since birth, Voldemort has been attempting to discover the path to immortality. During his time at Hogwarts, Tom Riddle learns about Horcruxes. Before he is sixteen, Riddle has realized that the only way to create a Horcrux is through the murder of another, as we learn from Professor Slughorn. This scene is meant to give the appearance that Tom is learning about Horcruxes from Slughorn, but in fact, as he is already wearing the Gaunt ring at this point, he has already made his first Horcrux. As Voldemort’s greatest fear is death, he thinks nothing of killing others to save himself later on, and creates his first Horcrux through the death of Tom Riddle, his own father. Voldemort spends the next several years of his life creating Horcruxes so that when the Avada Kedavra curse backfires on him, he doesn’t succumb to death.
In direct contrast to Voldemort’s desperate attempts to avoid death are the examples of sacrificial death. Both James and Lily Potter surrender to death to save their son. During the First Wizarding War, many members of the Order of the Phoenix were killed trying to stop Voldemort’s rise to power. Several characters are also killed throughout the books, many of whom give up their lives to save others. While those fighting on Voldemort’s side are willing to do almost anything to stay alive, those with the Order of the Phoenix, such as Sirius Black and Dumbledore, understand that their lives may be needed for their cause and are willing to give them up.
Harry’s attitude toward death is usually shown as a balance between recklessness and respect. While looking into the Mirror of Erised with Professor Quirrell, Harry is able to see the Philosopher’s Stone and retrieve it because he did not seek to use it to prolong his life. Though we don't learn this until the final book in the series, in the duel in the cemetery in Harry Potter and the Goblet of Fire, Harry is actually the stronger wizard because he is prepared for death, while Voldemort fears it. Many times, Harry’s actions have led to his being in a dangerous situation, yet he has faced death bravely. When he finally works through the riddle engraved on the Snitch, "I open at the close," he realizes that the Snitch can only be opened as he is facing his death. Using the Resurrection Stone Dumbledore placed inside, Harry recalls his loved ones temporarily from death, and from their company gains the courage to face Voldemort and his own death. It is his ability to willingly die to destroy Voldemort’s seventh Horcrux that actually allows him to survive. Following Neville’s beheading of Nagini, Voldemort is no longer mortally protected, and so his last, backfiring, attempt to kill Harry results in his own final demise.
Through the lens of family, the differences between Harry and Voldemort are reinforced. Harry Potter and the Philosopher’s Stone begins with Harry’s being placed with his aunt and uncle following his parent’s death. He never feels welcomed by his mother’s sister and her family and is treated as a second-class citizen. Contrasting that, Harry finds a place with Ron Weasley and his family. Almost immediately upon meeting Harry, Mr. and Mrs. Weasley show him love and affection and include him in their family. Harry ends up relying on this surrogate family for guidance and support.
Tom Riddle also grew up as an orphan: though his father was still alive, Tom was raised in an orphanage. Unlike Harry, however, Riddle never got a taste of what unconditional love was like, and never learned to trust others.
Harry’s experience with the Dursleys was apparently no better than Riddle’s time in the orphanage, though by watching the Dursleys' son Dudley and how the parents reacted to him, he was able to see directly what unconditional love could be like. One must wonder whether actually seeing the evidence of the existence of that sort of love would make the absence of it easier or harder to bear, though its absence must be made somewhat less painful by the apparent result it had on Dudley, making him self-centered and horrendously over-indulged. Yet, despite their similarly loveless "home" environments, Harry became a strong, moral member of the Wizarding World, while Riddle became Voldemort. Perhaps Harry’s parents passed on to their son favorable qualities that he could draw upon in times of personal struggle. Harry idealized his parents, while Voldemort looked upon his with disdain, killing his father as soon as he discovered who he was.