> For example, some people like to work on cars, but they don't get a job in that because their dumb uncle said this to them, so they spend their time grinding away at a fast food joint and going nowhere in life

I think the dumb uncle is correct. Although, if you like something well enough, then you usually become good at it, which in some cases means that someone will pay you for it.

It's up to you if you want to extract the fun and convert it into cash. It may be worth it, or it may be not.