This is a bit confusing. You seem to indicate he stopped having sex regularly at age 23, if I'm reading this correctly, as you say that he felt euphoria until he stopped having sex regularly at that age. I might be reading this wrong, but at any rate, it sounds to me like something happened to him at age 23. He might have had a terrible break-up that left him sexually depressed. It's not hormonal, I wouldn't think, because his body functions as it should, and at his age that's just not very common, it sounds more emotional to me. Or it might even be that he just got older and his sexual enjoyment changed. He might have been using drugs and stopped, dulling the experience, and didn't find a way to plug into that naturally. It might be when he was young he had sex with young women who looked sexy and didn't wear a lot clothing and as he got older those women didn't like him anymore or he wasn't meeting them anymore. Men aren't all the same. Just as many women as men are in it for the ego fulfillment rather than the relationship, so that's a myth. Yes, some men like the chase but as they age they might keep doing that but I know men like that and they aren't particularly thrilled being that way. But I also have known a lot of women who pile up conquests as well or go for rich men they don't really like or build a weak ego by having sex. It's not a one sex thing. I also think low dopamine is very rare. It's a bit of a scientific myth put out by pharmaceutical companies trying to sell medication, just as they did with serotonin. It's seldom true. You can burn it out by taking meds, though, or taking too many drugs, so is that something he does or did? And I agree, just because you think you're perfect for him doesn't make it so. Fact is, there is no perfect partners out there. People fall in love with lots of people over a lifetime, and they're all different and we love them in different ways and for different reasons. None of us has just one person out there, though we can talk ourselves into that and ruin a life doing so (I plead guilty). What I want to know is, assuming what you're reporting here is actually the truth, and the actual truth is locked up inside his brain so only he truly knows it, I still want to know what happened at age 23? Is he depressed? And frankly, if he is able to reach orgasm, it always feels pretty darned good even when you don't even like the person you're with. Maybe not euphoria, but how much in life is? Lots of questions, to me at least, and no answers given the little that has been said here and none of it by the person we're talking about. Peace.
The following article mentions the release of dopamine after having sex, giving a euphoric feeling. Maybe your friend could talk to a doctor and say he thinks this is missing and ask if that indicates anything physical.
Do you know this much about him because you're the girl? I'm trying to imagine someone telling something this intimate to a BFF; the love and concern in what you wrote sounds more like you're someone who loves him. I'm asking because if you're the "perfect girl for him" that you mention, I don't want to hurt your feelings by saying that he might have an idea in his mind of what constitutes an ideal girlfriend and the girl fits the bill, but maybe he just doesn't find her or the situation very sexy. For some guys, the thrill is in the chase, and once he has won the prize he loses interest. Or, they pick someone because she is pretty and has a background and education that is impressive, but ultimately the guy realizes she's not that exciting to him even though she checks all of the boxes to his family or his sense of what an ideal girlfriend should be.
The other thing is, if he can feel good and ejaculate, it might be asking a lot to always have "euphoria" along with it. Especially after a couple of years in a relationship, the froth of courtship goes down, and a longer-term love takes over, and it's not really characterized by euphoria.
But if he's saying he doesn't find *any* woman particularly sexy, then that's another thing. He could go to his doctor and get his hormone levels checked.