What do I really owe the people I mentor?

You want to mentor well, and you are honest enough to notice that your own interests can get tangled up in it. It feels good to be looked up to, useful to have loyal people coming up behind you. Wanting to sort the real duty from the part that just serves you is exactly the question a good mentor asks. What you owe them is their growth, even when it does not benefit you. The test is simple and a little uncomfortable: are you guiding them toward what is best for them, or toward what keeps them useful to you? A true mentor helps a person outgrow their need for the mentor. Someone serving themselves keeps the other person dependent, maybe without meaning to. Try one thing that costs you a little. Push someone you mentor toward an opportunity that helps them even if it takes them away from you, a role on another team, a chance elsewhere, a skill that makes them less reliant on you. When you can do that freely, you will know the duty is winning over the self-interest. That is the whole art of it.