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?
There is a distinction between miners and nodes though miners will (or at least should) run a full node to check that the blocks they are attempting to mine are valid blocks. I think you are using the terms "miner" and "node" interchangeably here. There are many nodes (majority) on the network that just validate transactions and blocks and don't attempt to mine blocks.
Why do nodes gain in propagating the transactions they receive?
Full nodes and miners want to receive information on the latest transactions and blocks from peers. And hence effective propagation relies on software that can both send and receive this information. In theory the software could be altered to only receive and not send. But if this became a serious problem on the network some nodes could choose to only maintain connections with nodes that are both sending and receiving and the "selfish" nodes might struggle to maintain connections with peers.
The nodes run software which contains the best-for-everybody rules about how the network should operate. One of those rules is that nodes should forward new transactions to other nodes.
There is no incentive for them that I know of, other than, to not do so, would risk not participating in the network of nodes that do follow the best-for-everybody rules. By not participating, they are spending their own resources without being able to participate in the network's rewards.
And as another commenter mentioned, they would be made to not participate, because the nodes that are following the rules would eventually ignore them.
Mining bitcoin is based on game theory. Play by the rules and get financially rewarded. Selfish miners risk not making a profit and damaging the coin's reputation. And alternative node software gets squeezed out by the majority because nodes/miners follow the longest chain. Not sure if this answers your question?