You have to live a long time to see the full curve of friendship. A while ago, we used to have a thing called a Rolodex or an address book. Remember those? You had a compendium of addresses and phone numbers of most people you knew, but maybe you put that antique way of remembering contacts away when the world went digital. If you did, but you picked it up again, you might find it interesting to see which of those previously well-used numbers and close friends are still friends. I found this exercise fascinating, but not surprising.

Looking back, I see how many of those friendships were acquaintances, molded by proximity and mutual need. We were neighbors, or our kids went to school together or we were on a charitable board at the same time. Good, even great, people almost all of them — but most are essentially gone from my life.

Some, however, have remained my dearest friends for more than 50 years. I still see Lauren and Andy, women I’ve known since kindergarten in Chicago. I have an enduring, familial relationship with Janet, whom I knew as an undergraduate at Washington University in St. Louis and who became my lifelong friend and collaborator when we both went to Yale for graduate school. I have more than a dozen friends I met here in Seattle, friendships that run from decades to half a century or more.

What is it that not only creates an eternal place in your heart for some people, but also makes them love you enough to reciprocate — enough to keep love alive? What is it about friendships that keep that love vivid, even after people have died? Is it so different from romantic love? Or is it just someplace lower, not erotic, but no less profound, on the spectrum of attachment?

I can’t always explain it. It is like physical attraction — powerful but not entirely understood. Still, I do know a few things about friendship. That is, true friendship. A relationship is trivial if it is not honored. You have to show up and be there for someone, either in person or on the phone. You need someone who at least occasionally shows up for you. You seek their presence, find them worthy as people, exchange joys and sorrows, and can handle and accept their flaws.

You feel truly known by them. You know they want the best for you, and you will do what you can to comfort them or celebrate the high points of their life. You can feel when you have not been with them or talked to them enough, because you do not want the connection to be broken.

You don’t want this to happen because you know that like romantic love, a deep friendship has to be nurtured by presence as well as history. You may not consider it tragic if it fades into acquaintanceship, but like love, was it really love if the loss of it does not pain you?

True friendship is sacred. It requires a lot of me, and if I disappoint myself in performance or awareness, I hold myself accountable and try to do better. Honestly, I ask a lot back. It’s never a quid pro quo. We should not, and cannot, require everyone to give in the same way or even the same amount. But over this long arc of life, true friendship does require continuing consideration of the relationship, and if it is put on automatic pilot, it will, like more pedestrian technology, eventually crash into something and cease to exist.

It’s important to add, however, that it really doesn’t always take extraordinary measures to keep a friendship alive. On Valentine’s Day, I generally get electronic hearts and sentiments on my phone or computer from some of my friends. It’s not as literally tangible but nonetheless I do not delete these expressions of continuing affection. A reminder, in any way, that my friend is thinking of me, is enough.

But over this long arc of life, true friendship does require continuing consideration of the relationship, and if it is put on automatic pilot, it will, like more pedestrian technology, eventually crash into something and cease to exist.

Do you think friendship is possible after a dating breakup? My ex-girlfriend and I broke up because I was not ready to marry her. I do prize her as a friend. Do you think it is possible to convert that romantic relationship into a friendship? I care for her but we are too different to be together, and I just don’t have romantic feelings for her.

This is a tricky situation, but not an unusual one. Many relationships break up because of unequal or different feelings. You left because she loved you and you liked her. You still feel the same way, but she is bereft because you made it clear that there was never going to be a committed and romantic future together. So, you are where you are emotionally and you would like her to come to the same emotional space.

Can she get there? Probably not for a long time, and maybe never. The best shot for a change, however, is if she falls in love with someone else. If she finds someone new, and is truly in love with them, it might make her rethink her past relationship with you. And over time, she may be able to think of the things that made you close as part of her history that she can reclaim without feeling rejected or sad. But that is a big “maybe.” It can take a long time to change emotional feelings, and some people who have loved deeply just can’t make the transition.

So, here’s a path that might work. Wait until you know she is with someone else. Then call or email (texting might seem too trivial) and tell her you miss her friendship. It would be best if you wait until you are with someone else you are serious about so there is no mistaking your intention. You owe it to her not to open the door for hope. It has to be clear that friendship is what you want.

If she has changed her feelings about you, she might answer and accept your offer for coffee or a walk or something like that. If she does, you will have to judge if she has really reclassified you as a friend and not as a potential husband. Just be sure you do nothing to rekindle her romantic feelings, and if you sense they are there anyhow, it’s best to back away. If you really like your ex-girlfriend, you don’t want to torture her if she is still in love with you. Sometimes it takes years, if ever, for love to turn to like.