Honestly, I think enough is trying to make sure their lives are better for having me in them. Am I doing enough, am I contributing in a way that makes their lives easier or better or more fun or nicer? My life is generally fairly under control so I have extra capacity to put effort into others...
... that said, I have had friendships where the other person needed way more from me than I was able to give, and they've either blown up or fizzled, which is fine, friendships evolve.
I have had girlfriends that wanted way more from me than I was willing to give, and I think part of the whole dating process is figuring out how independent or self-reliant you both want to be... it's part of the whole search for compatibility.