mining theory – Trust in bitcoin networking

I am new to the subject, but as I was studying I wasn’t able to understand something and I would appreciate if anyone could help me.
I understand that in most aspects bitcoin doesn’t rely on trusting the nodes. It’s in the best interest of each node to validate and append blocks broadcasted by other miners, so we can assume that the nodes will do it. It’s in the best interest of each node to validate the transactions they put in a block, otherwise it will be rejected. But why do nodes gain in propagating the transactions they receive? If that’s not how it works, how is the transaction poll created?