I get that feeling often. I think it's the process of pulling/seducing a girl that causes the positive emotions, rather than achieving the goal in of itself. You get more excited with every step that gets her closer to your bedroom, and once you orgasm, all of your goal-oriented behavior for the entire night is basically eliminated instantly, and you're left wondering why you put so much time into sticking your dick into a wet hole. The only time I feel good after sex is if I actually like the girl enough to want to still spend time with her or if the sex itself was an orgasmic psychedelic mindfuck, which is rare.
People get confused about motivation and positive emotions, as far as I understand the dopaminergic reward systems in our brain make the pursuit of a goal pleasurable, not the actual achievement of a goal. That's why people who complete their PhD thesis can feel utterly depressed once they're done, because they're suddenly left with a feeling of "now what?" It's the same with eating a meal, you are super excited when you first start eating, but once you're done you might feel like shit because this dopamine-mediated pathway is now suddenly eliminated and your brain has to figure out what new goal oriented behavior to identify.
People get confused about motivation and positive emotions, as far as I understand the dopaminergic reward systems in our brain make the pursuit of a goal pleasurable, not the actual achievement of a goal. That's why people who complete their PhD thesis can feel utterly depressed once they're done, because they're suddenly left with a feeling of "now what?" It's the same with eating a meal, you are super excited when you first start eating, but once you're done you might feel like shit because this dopamine-mediated pathway is now suddenly eliminated and your brain has to figure out what new goal oriented behavior to identify.