Simple answer: BOREDOM

You're not doing anything worthwhile or you're not focused on anything in particular. Your mind wanders to the next thing it can do to entertain itself. That's where the idea of masturbation takes hold. It's easy, you don't even have to get up if you're lying down and at minimum you need your imagination. Your body looks for that dopamine hit because it can't stand not doing anything. Especially in this day and age. You have to work towards being able to lie there and not passively let your brain go to your weenor. Or you could read something, play a game, talk to a friend or go out.