Habit.

Unless I'm flat exhausted or upset about something, I almost always masturbate right before I go to sleep.

The funny part is, if I had sex that day, if I had sex that night even, it doesn't reduce the urge at all. I've lain awake next to men I just fucked, wondering if it would be weird if I snuck in a quick wank. Not because it was unsatisfying, and not really because I'm superhumanly horny either. It's just no reason to break the habit.