I’ve taken to telling them that someone else helped me out, and I’m just trying to pay it forward, with interest. If they can pay it forward in future, I would be very happy with them continuing the chain.

You still get the dopamine of a good deed, they feel like they can pay you back by passing it on. If they don’t, no harm done. If they do, your good deed just got amplified to some other poor sod’s benefit. It might even loop back!