Christianity and capitalism. If it doesn't make you feel guilty the Christians don't like it and if you can provide it to yourself for free the capitalists don't like it.
So, like, for the bulk of history, the people demonizing it are religious assholes.
They demonized sex out of wedlock, demonized wanking off; and any other kind of sexual release, while simultaneously deciding who you can marry (and therefore have kids with,).
It’s one of their core methods of social control, ensuring wealth is only passed on to children of wealthy and “faithful” families.
Masturbation is totally normal and healthy, and you're spot on that it shouldn't be demonized or shamed. In men, it might even reduce the risk of prostate cancer.
So while I'm totally with you that masturbation is healthy and that bullshit social taboos against it should be rejected, it's also good to be mindful about your motivation behind doing it. Are you doing it because you're escaping pain? Or are you doing it because it aligns with your values and makes your life meaningful? If you rely on masturbation too much and don't have ways of accepting your emotions and connecting with the world, it could potentially tip into unhelpful psychological rigidity and a frustrating life. The key is to be able to experience masturbation while still staying flexible enough to show up fully for the rest of your life too.
Because you enjoy it. If you're fixing your issues, it must be through pain and suffering. If it doesn't involve pain and suffering, then it isn't fixing your issues. The "Protestant work ethic" doesn't come right out and say that, but it's the implication.
See also: denial of LSD and psilocybin for mental health purposes.
Because easy dopamine hits are also easy to gain unhealthy addictions to. Because it's literally right there, easily accessible, at all times.
Couple this with how incredibly unhealthy the social relationships portrayed in most pornography are, and you're gearing up for a lot of young men addicted to wanking and having unrealistic expectations of sex. The porn young women read isn't necessarily much better, in the regard of healthy social sexual relationships.
Now, I'm not one of those weird "you shouldn't jerk it at all" folks because that's just extremism in the other direction.
I actually asked my family doctor at one point about the health effects of masturbation. She said that as a guy, if you are not otherwise sexually active, it's good for the prostate to keep the plumbing working down there.
Religion, capitalism. Powerful groups want more people to have more children.
Luckily I'm in a progressive enough country that even in school we were taught that masturbation is a thing and not necessarily bad.
As for negative effects - if you do it TOO much, particularly with a very strong grip, then don't be surprised if, when having actual intercourse, you're just not feeling much and might be unable to reach orgasm. You might even be uninterested in your partner sexually. A few days without masturbation will fix it though, doesn't seem to be permanent. Day 2 without doing it and I couldn't keep my eyes (or hands) off my wife's body.
Sex was very infrequent for me and my wife in the last few months of her pregnancy, so that's how I know. Soon as we started doing it on a somewhat regular basis again, I opted to quit jerking it because I wanted to enjoy the real thing more, even if it's not every single day. No long-term negative effects that I've noticed.
A lot of religion has been to push a heterosexual couple together for the means of procreation. Masturbation has been seen as a way for people to lessen their urge to procreate in the appropriate canonical manner.
These days mutual masterbation is better for your relationship than having kids. It's not the kids fault, society has made having kids a nightmare.
And of course the reason it is demonized is that any powerfull organization/society needs peoples shoulder to stand on. So, the more people, the more power. And they don't really care if it was by rape due to sexual frustration, they just need more people to take advantage of.
You realize things aren't viewed the same all over the world right? Here in Sweden it's nothing bad or wrong, we generally have good sex education and parents that are fine with it as long as we keep it private and clean. And as adults it's completely normal, not that uncommon to talk about either. There was a monthly magazine for teens when I grew up that talked a lot about sex, sexual identity and stuff like that and the readers could send it questions to get answered by professionals or other readers. Very open and helped so many with things they didn't dare ask parents or others about and it was always a better source than the Internet when that came around. Pretty sure it's still a thing too.
So it's just seen as a thing everyone does and enjoys.
I think the simpler answer is, you are so right, it barely has to be valorized. It sells itself.
I think the long answer is that porn addiction is a real thing that can affect people. Also lots of people in charge of things like sex education have historically been prudes who see masturbation as some sort of gateway to promiscuity.
Easy dopamine isn't a good thing. Dopamine is finite but renewable, so if you run through all your dopamine on easy hits your going to end up having motivational issues.
And if your constantly chasing short term Dopamine hits your brain is going to adjust to seek behavours that give instant gratification over somthing that's just as rewarding, more productive for your personal or professional life, but takes longer to get that dopamine.
By all means deal with your libido in anyway that doesn't hurt the people around you, but maybe retiring the wankers cramp for a bit might do you good once in a while.
Often times it's about control over people. Whether it's religion or capatilism sense it's something you can do yourself for free that gives you pleasure. Capatilism can't force you to pay for enjoying yourself and religion doesn't want you to do things that they don't control
Religion is mostly to blame, but I think some of those religious teachings have been integrated into institutions and secular society as well. Notions that self gratification is a treat and not a need, the perception of masturbation that it's some teenage boy temporary phase and not for women or older married men. The idea that it can't be a social activity at all, even though it's free and safer than drugs.
One major reason is that feminism, which deeply influences culture, posits that all men are rapists in the waiting. For an example here a quote from a prominent and influential feminist.
"Under patriarchy, every woman's son is her potential betrayer and also the inevitable rapist or exploiter of another woman."
Andrea Dworkin
This chills any frank discussion of male sexuality because that would be implicit endorsement of sexual assault.