A recent study on brain patterns shows that people who think they are playing against real people try harder, then when they believe they are playing against computers.
Granted the study used gambling games to monitor brain activity, but I think it could also offer some interesting insight into what motivates video gamers as well. At the very least, it reminds us why we often care more about beating or losing to a group of online players than dying in a single player campaign.
This was more pronounced, not surprisingly, in men than it was in women.
Women may not have been as engaged playing an alleged soulless computer. Furthermore, male and female subjects always believed they were playing a male contender in the 'human partner' tests. It has been documented previously that men and women play games differently in the presence of a male partner". However, in this respect more research is required to give any definitive answer to this question.
It would be interesting to see what a similar study would show in online video games, like Call of Duty or Killzone 2.