"Life is a merry-go-round" is somewhat negative. If I say something is like a merry-go-round, it means that there is some repeated cycle that occurs again and again. It is often implied that the activity has little or no purpose. If two countries were in negotiation for years, with no resolution, I might talk about a diplomatic merry-go-round. "Life if a merry-go-round" can be taken to mean that life is cyclic - a person might also take it to meant that it is somewhat pointless, depending on whether the glass they are holding is half-full or half-empty (how cynical they are).

"Life is like a merry-go-round" means life is always changing.

I think it is like a caroussel. It means it spins and spins.