I was thinking, and I came up with the idea that when I'm happy, I'm doing something that I like, and I don't want that the moment of happiness ends, so after the moment of happiness I feel sad. One example is when I create an ideia, I go to the university where I work, and I discuss the ideia to the students after the class, and I'm old now, and I am still learning, that is fantastic, so after class, I feel sad that the moment is over, so I go to my house to work on the ideia to discuss it again. So the conclusion is that when I'm sad, I want to work on ideas to feel the pleasure. Is this an auto motivating way to do something that I like? Can we see this in the humanity? Is it just like Einstein when he dedicated his time to physics, not for other subjects, he used to go bad in other subjects at list (as far as I know).
Well, if I’m Always happy, doing what I like, one day, I’ll not feel the same plasure as before, like when you used to eat a lot something when you were just a kid, and now you don’t like it anymore, so, in my opinion, is not good to do everything to much.
Some psychologysts and some philosophers don’t say “sad” but they say that you miss doing something that you like, I partirculary say sad.