I can't say I chose it. It just felt right for me. When I became one is hard for me to answer because I've never been religious. So in a sense I guess you could say I've been an atheist my whole life.
I'm a very logical and liberal person so believing in my self, science and the truth feels right for me. Stuff like us having an afterlife and the possible existence of a 'heaven' is a comforting thought, that I may see my dog and a very good friend of mine (who died in a car crash a little over 6 months ago) again. But when I think about it seriously I don't believe in an afterlife.
I find it very strange that someone would have the need to believe in some higher being and for example creationism in the 21st century when so much has been explained with the help of science in the last 100 years or so.