Bilbo Baggins a hobbit of the shire, is Frodo's uncle. Bilbo is not like the other Hobbits, he longs for adventure in his life and so on his birthday he sets out to go and visit the elves one again before he gets too old. Bilbo turned adventurous after being friends with Gandalf the Grey!.
As the film goes on you begin to see Merry regretting his choices to come on the quest but realises that if he does not carry on then there will be no shire for him to go back to.