From some reading with the kids yesterday in The Hobbit:
[Bilbo] did not, or course, expect that any one would remember that it was he who discovered all by himself the dragon’s weak spot; and that was just as well, for no one ever did.
(chapter 15, “The Gathering of the Clouds”)
Tolkien describes a character quality here that I can hardly imagine. Probably the top reason for this attribute is that Bilbo’s chief motivation, in all of his most heroic acts, is the thought of home (and of breakfasting upon eggs and bacon). Bilbo embodies the Ciceronian ideal of a bold public servant who longs only for the quiet life of his country estate.