I've learned to identify those people and take them for what and who they are. I enjoy helping them, being close to them, and learning from them and they in turn are usually quite grateful for any help I can provide. If I notice that it *is* in fact taking me for granted that they do, then I cut them out, even before they themselves leave.
But I'm aware from the beginning that they will at some point leave, or at least be less intense with me. And I'm fine with that. It's their need that created the intensity and once that need has been fulfilled it automatically diminishes. Sure, it's always a bit sad when that does happen, but I'm happy to have the memories and the experience and I'll gloat a bit that their life is better becoz of me
And on occasion, you will find the odd individual who will prove to be a special person, who you are so in sync with that they are worth holding on to, and usually, they'll feel the same about you. Those tend to stick around no matter what and become your best friend. They'll be the ones that will help you once you've helped them, as they sense *your* need. And they'll be a source for personal growth for you as much as you are to them. Those are the actual connections that are worth keeping.
Friendships have their purposes and their expiration date. Only a handful of 'em will last a lifetime as their purpose is neverending. Finding those people though...not always easy