I have a weird mental thing going on when I commute.
If there is another cyclist ahead of me, even a block away, I want to catch and then pass him or her...
To do so isn't necessarily a bad thing depending on the reason behind it. I see other other cyclists or cars as a goal/target (as someone else said, a "motivator"). When reaching that goal I look for another but never is there an "I win" moment. In order to win you actually have to enter a race.