Ironically this probably isn't only because doing things brings happiness, but because more experience means more context, and more context means more understanding. The only way to gain a better understanding of your life is to have more types of experiences, sitting there and thinking about it within the framework of the experiences you've already had generally isn't that useful.
E.g. the best way to understand why a previous relationship didn't work out is to have more relationships.
This ignores all the people who are leaving a trail of destruction in their wake without much self-awareness or guilt.
What does that mean? Sounds interesting.
Everyone I know who says shit like that publicly is like that. Most people don't share stuff like that unless they are assholes.
then
> The people I know who do improve
Despite the contradiction here, it sounds like some people do find something on their quest, and that thing is to start taking real action.