While happiness might need reason, life doesn’t. I find that, in a way, we live in a probabilistic universe with enough attractors that allowed things to form. Among them were humans, now also building some things with/against the odds, and subsequent self-image/sense of importance.

You can still suspend thinking about the inevitability of death and inherent lack of meaning to feel or create something. It does require one to choose and get comfortable making choices that are beyond right and wrong (not in a moral sense), however.

I don’t know if there is one answer for why people can still feel happy despite it all, and I suspect there will be different reasons. One reason could be that they’ve just accepted the futility, focusing on what makes them happy. Or maybe they’ve accepted that pursuing universality/objectivism when it comes to subjective things is impossible. Or maybe even that no matter which option one takes to view life, one cannot escape delusions.