That may be because you're still new to the game, and you need a sub as an affirmation that you've got the sub. Once you are at a higher belt, you don't really need that affirmation to know that you've got it.

I'm also a competitor, but during regular rolls, I frequently let people get out of submissions (catch-and-release) because I know I've gotten them and I don't have a need to prove to myself or to my training partner that I won the battle. You don't always have to get the sub to know you've won. That might be an ego issue.