I keep hearing about how great mentors are, but what's in it for the mentor? Why should someone who knows what they're doing provide such a valuable service fo

I keep hearing about how great mentors are, but what's in it for the mentor? Why should someone who knows what they're doing provide such a valuable service for someone who can't pay them back? In older times, tradespeople literally got a live-in servant for the trouble of teaching them, but other than college tuition I haven't heard of a good compensation model for mentors.