on a few teams' websites they have a feature going where a player talks about why they wear their number...
most wear it because someone else wore their number, it was a number of their idol, or it was a number worn by someone in their family, or a combination.
http://www.nba.com/magic/news/behind_the_numbers.html
this is the page on Grant Hill, but there are others like Ginobili, Earl Watson, Gooden, Bogut, Pierce, Piatkowski! and more.