The answers so far have been thought provoking. Reading them, I'm reminded of some old psychology classes where I first heard of Maslow's needs hierarchy.
For those unfamiliar, Abraham Maslow became renowned for his theory that people are motivated by a hierarchy of needs. Maslow described five tiers of needs beginning at the lower end with survival and security (basically food, shelter, and a reasonable expectation of safety). Once these needs are satisfied, people are free to satisfy their "higher" needs for social interaction, personal esteem, and self-actualization. Maslow theorized that people were not free to satisfy their higher needs until their lower needs had been met. (Although Maslow's theories are frequently cited, they remain just that--theories--as they have never been empirically validated.)
Very few of us play at a level where cash is a significant motivator. Many play for the game's social aspects-- particularly league players. At this level of motivation, it's not necessary to play very well. (In fact, it can be a hinderance, as casual players are often put off by those who take the game "too seriously.")
Not surprisingly, the dedicated players who typically frequent this forum are motivated by the higher level motivators of esteem and self-actualization. "Esteem" in this context closely relates to the "ego" motivation cited by the OP. Several of the respondents so far have admitted that ego does (or once did) act as a significant motivator.
Most of the respondents, though, are clearly working at the highest level of motivation, self-actualization. I can remember several discussions of what exactly Maslow meant by this term, but I don't think it can be understood any better than by simply reading what Greyghost wrote in his answer.
As for myself, I'm probably at this motivational level as well, although I don't play nearly as well as most here. (Never picked up a cue until I was in my 40's.) I enjoy learning and working on technique such that I can hit some of the advanced shots that the top players play, even if I'll never "own" them.
Let's keep this thread going... its been interesting reading so far.