[Raefindel]

I believe there is also much to be said for family history. Gandalf knew "The Old Took" and had an adventrue or two with him. Pleased with the results and seeking similar qualities in another, he would naturally seek them in a relative.

Bilbo's Mother was a Took. It says that they believed a Took, somewhere down the line, had a "fairy wife" and that is the reason given for the tendancy for adventures.