Like most of you said, personalities are shaped a lot according to surroundings, experiences, etc.
I currently RP a Lumberjack, a Food Distributor, a stone miner, a wanderer, a wierd Preacher-type guy, a political liberal, and a few others.
It is good to give characters at least one attribute that will make them unique. For example, I have one character that is attributed by extreme sadness. This makes some pretty interesting role playing.
All in all, I have to say that if you play a roleplaying game, the purpose is to ROLE PLAY, not make a bunch of characters that think, talk, and act just like their creator.
