Whenever I feel really depressed, I turn to Marcus Aurelius' Meditations to brighten up my day. Marcus Aurelius was Emperor of Rome in 161-80, and a philosopher to boot. Being Emperor wasn't as much fun for Marcus as you might think. Some examples of his happy thoughts:

Begin each day by telling yourself: Today I shall be meeting with interference, ingratitude, insolence, disloyalty, ill-will, and selfishness.

- Meditations, Bk. II, Para. 1.

[L]ook at the characters of your own associates. Even the most agreeable of them are difficult to put up with; and for that matter, it is difficult enough to put up with one's own self.

-Meditations, Bk. V, Para. 10.