It is strange that friendships, which nourish and sustain us and often provide our deepest source of connection, lack tvingande länk the sort of standards that are routine in romantic relationships

I have this friend, Sarah. Since meeting in our thirties, we’ve shared many of life’s essentials: hairdressers, dog-walkers, phobias (airplanes and mice), health scares, worries over our kids, and insomnia caused by husbands who snore. But lately I’m aware that whenever Sarah calls I feel a tightness in my chest and, more often than not (thanks to caller ID), I don’t pick up the phone. I feel guilty, but that’s preferable to spending hours listening to Sarah complain. I’ve been meaning to tell her how I feel, but I haven’t quite worked up the nerve. Most of the time I feel like a bad boyfriend.

We became inseparable and, at one point, I secretly tried to find out if it was possible to be adopted by your best friend’s family if your own parents were still alive. It wasn’t until college and postcollegiate life on opposite sides of the country that we drifted apart. But we never lost touch and, years later, when I moved with my husband to the city where Natalie lives, she seemed thrilled. She threw a dinner party in our honor and did everything possible to make us feel at home. Then, after about six months, Natalie suddenly stopped calling, and whenever I tried to make a date she claimed she was too busy and got off the phone, fast. To this day-ten years later-I have no idea why she gave me the boot. Now when our paths cross, we greet each other like distant acquaintances and I feel bruised all over again.

If your significant other stops calling, makes impossible demands, or treats you like roadkill, you deal with it. It may not be easy-you may put it off-but eventually you’ll find out where you stand. Not so with friends.

Then there’s Natalie, whom I fell in love with when I was 9

“You don’t get together and say, ‘I’m really mad at you, I’m not going to see you anymore,'” says Ruthellen Josselson, PhD, a Baltimore psychotherapist and coauthor with Terri Apter, PhD, of Best Friends (Three Rivers Press). “To the extent that we have a ritual, it’s not calling, not getting together. But that makes it difficult to know when someone is distant because she doesn’t want to be your friend or because something’s going on in her life that’s keeping her from being in touch.”

So how do you know you’re being fired? And what do you do when you’re at your wit’s end-as I am with Sarah-and ready to issue a pink slip of your own? “It’s a complicated dance. We start learning the steps when we’re quite young, and they don’t change all that much,” Josselson says. If nobody calls or makes a move, if you run into each other and say, “Let’s do lunch,” but don’t, if one person is suddenly booked until 2013, sooner or later the message gets through.

Luckily, most friendships have a natural life cycle. Often we’re drawn together by circumstance-work, the single life, kids-and as our situations change, we gradually drift apart. On a deeper level, our friendships mirror our internal life. “As we gain a stronger sense of self, what used to matter no longer does, and we’re bound to outgrow certain friendships,” says Florence Falk, PhD, a New York City psychotherapist. “Once you’re aware of that, without being cruel or feeling guilt-ridden, you can begin to let go of relationships that no longer nourish your most authentic self.”