I think it might be due to the fact that after you accomplish something that you really wanted for quite a while and/or that you prepared a lot for, you can end

I think it might be due to the fact that after you accomplish something that you really wanted for quite a while and/or that you prepared a lot for, you can end up realizing that while it was a great achievement, you are still the same person with the same habits and issues, and that the achievement doesn't really change anything outside that part of your life, and sometimes there is also no reason to think you can replicate the achievement itself. And then you realize that you are objectively really lucky and can't even properly appreciate and enjoy it, and that brings you down even more. Even people who never had any great achievements can experience it with sex: sometimes you are alone for a while, and think how amazing it would be to have sex with someone, thinking about all sorts of happy feelings you'd have and how you would be happier overall and so on. Then you have a one night stand, and it's good, but you realize it hasn't really changed anything, that a relationship might not necessarily work out, that you are not necessarily going to be able to repeat it, then think about how you got exactly what wanted and that many other people would have really wanted to have your experience, and yet you can't appreciate it like you think you should. I think the solution is partly to accept it, and just let your expectations about the happiness related to those achievements go down, and also trying to focus on achievements that have lasting effects, mainly those that involve getting wealthier or healthier, and finally trying to have a more balanced life, relying less on high-impact random once-in-a-while experiences and more on habitual routine experiences, although it can be hard to find the motivation for that for some people.