The incentive works in a kind of round about way, where the more nodes you talk to the more nodes you know and therefore the more fast/high capacity you know an

The incentive works in a kind of round about way, where the more nodes you talk to the more nodes you know and therefore the more fast/high capacity you know and can choose from. So sharing more bandwidth leads to you knowing more nodes, which means you'll have better tunnels. So there is an incentive for you to share more.