Solving people’s problems, says Knuth in this interview:
Knuth has been called the father of the Analysis of Algorithms and he explains his decision to embark in this particular field. He says, “If I consider the entire class of all interesting algorithms, then it’s bound to be full of problems just as interesting as queuing and hashing…So that’s why, right at that point, I said ‘Hmm, that wouldn’t be bad…to spend a lifetime on it, because you have a huge number of problems, not only do they have beautiful mathematical structures that tie together, you know, hang together in nice patterns, but also there are customers out there; so that when you solve the problem, the people say, ‘Hey, thanks for solving the problem, Don.’ So it’s a great field to embark in.”
From Dr.Dobb’s Donald Knuth for Posterity.