We've all probably seen the Full Tilt Poker commercial "Why We Play" and that brings up the subject as it applies to our group and everyone we play against both live and online.
I am sure that there are plenty of different reasons why everyone plays poker; the challenge to outplay someone, the drive to amass the largest stack possible, the social interaction with other players, and many more. Furthermore we are all a mixture of the different aspects to varying degrees. For instance, I'm sure nobody wants to leave stuck for the session but I'm also positive that some people care a lot more about their wins or losses than others. Some players don't sweat losing at small stakes and other will find it as distressing to lose a buy in at microstakes as they do at larger stakes. Some poker players treat a buy in as a night's entertainment and figure if they leave empty handed after playing all night then they got their money's worth while still having a chance to come out ahead which you couldn't expect from going to see a show. Then there are some players who would rather chase the dramatic suckout against all odds because they will get more gratification out of catching runner runner than they would folding because it is mathematically correct to do so.
I don't fault anyone's motivations and I'm certainly not going to try to change them. As long as they are getting what they want out of poker and only tapping appropriate funds then more power to them. But whatever the reasons for playing, they are going to be reflected in the decisions at the table and why they play is something to consider when optimizing play against another player.