People play games for different reasons at different times.
Richard Bartle was one of the first to present a coherent theory about this - the bartle test. Another researcher, Nick Yee questioned Bartles findings in his daedalus project.
Bartle initially started with 4 player types below:
Achievers
Explorers
Socializers
Killers
Bartle increased his graph to 8 types. He discusses these types at length in this article on his website :
source for above content.
Nick Yee had a different perspective than Bartle, and presented the following reasons play mmorpg's source) :
I think people probably are generally one specific player type, but drift into other types as they play.