In encryption, the weakest link is almost always a person.
While you could spend many hours attempting to decipher an encrypted message, or intercept a password, you can easily trick a person into telling you this information.
Suppose Bob works for a large company and encrypts document E with key K. Suppose Eve, wishing to decrypt document E, calls Bob and pretends to work for the company's information security department. Eve would pretend a problem existed with the computers, servers, etc. and ask Bob for his key, K, which she would use to decrypt E. This is an example of social engineering.
Randall Munroe in an xkcd comic once presented a scenerio in which bad guys find it more convenient to hit Bob with a $5 wrench until he gives up his key rather than attempt to break the crypto system.