Matthew Baldwin wrote in his blog Defective Yeti, "I think the secret to happiness is to care a lot about people who care about you, and to not care too much about anything else.". (link)
Intriguing thought. I think a key part of it is, it's not black and white. If it said "I think the secret to happiness is to care about people who care about you, and to not care about anything else.", then I'd have to way totally disagree. We shouldn't just care about those who care about us. We should care about all people, at least a little. And certain things are worth caring about too. Like the environment for starters.
But, yes, it makes sense to me that the biggest dose of caring should go to those who care about us. People are important. Our relationships. More important than things, I think. And, when it comes to people, yes, those where the caring is mutual are worth the most thought and concern. They merit the highest emotional investment. Or, put another way, we are less likely to get hurt caring about those who care about us.
But when we care too much about those who don't care about us, or who just don't care as much about us as we do about them, well, I've experienced the hurt of caring way too much about someone who didn't care as much about me. It can ache pretty bad. It's hard to be happy when hurting deeply.
That doesn't mean we shouldn't care about those who don't so much care about us. It just means, as I see it, limit the emotional investment. Don't invest yourself overly in someone who's not going to give back.
But do though, show kindness and consideration, and really, I guess, respect, to all. And an openness to friendship. Someone you care about only a little may prove worthy of becoming someone you care about a lot.