I believe all the luck references are building to this final quote at the end of the book:
"Then the prophecies of the old songs have turned out to be true, after a fashion!" said Bilbo.
"Of course!" said Gandalf. "And why should not they prove true? Surely you don't disbelieve the prophecies, because you had a hand in bringing them about yourself? You don't really suppose, do you, that all your adventures and escapes were managed by mere luck, just for your sole benefit?....you are only quite a little fellow in a wide world after all!" (emphasis added)