If you were born into a very tough situation in a war torn or third world country, you'd do what you needed to with what was given to you. While we do have a lot of material wealth and might have more free time, we still face the same great equalizer: death. As do our civilizations and our planet (and likely us, way before that). Given the constraints, you can simply "try your best". Living without regrets might be a job to one person, a 90 hour startup to another, traveling and learning to others. Whatever your belief is, following it is the best that you could do. Ultimately you have to answer to yourself & be able to live with yourself. Personally, living selfishly, while it maximizes certain things simply goes against my core beliefs. I don't necessarily know that living less selfishly is necessarily better. But for me, it is. Your beliefs, may defy logic, and may not necessarily maximize legacy, wealth or whatever, but in having acted you will have done your best given the situation to be able to live with yourself, which is all you can ask of yourself.
If you have to be less rational to accept an answer, doesn't that imply that the answer is wrong?
Oh, wait, everyone with a scientific turn-of-mind has already been labelled a "robot". Beep boop, then /s.
Why do people want to make a lasting change? Purely for intrinsic reasons? Or is there some hidden assumption like for example that it will make them happy in their later years?
Yet you seemed to want to draw a distinction between happiness and a lasting impact. Though the distinction was perhaps just with the family/local impact.
> I also don't think there's some kind of "altruism" that isn't ultimately just another way of getting happiness - maybe a more intellectual one, out of the feeling of doing something one considers morally good.
Right, but a family-related happiness is less because it's just a result of chemicals (oxycotine?) that are related to survival, a sort of failure mode? You said yourself that it is less fundamental. What is fundamental? And why is your derivation of happiness higher than those who go by the supposed more base instincts of focusing on their family? Because the hormonal causes are harder to pin down and analyse? So what? Something more complex and harder to describe is better?
If you feel happy, I think the distinction of why is unimportant.