Last week I traveled to Buffalo, NY, to visit friends from high school. These ladies were not those I considered my best friends for much of high school. We didn’t have a ton of classes together and we didn’t engage in a lot of activities after school together, either.

What made these relationships different was that when we went away later in life, to college, to jobs, to other cities, we always found ways to make time for one another.

So now, 15 (yup, that long!) years since we graduated high school, we still make time for one another at least once a year. I live in Nashville and my ladies live in Buffalo, but they come down here or I fly up there. My parents no longer live in Rochester, where we all went to high school, so that makes visits more difficult because we don’t have the holidays to share with one another.

When I started this post–two weeks ago as I was thinking about my journey to Buffalo–I was thinking about how the time and resource investment is what makes these relationships special. There are friends of mine from that same period in my life that are visiting Nashville this week, but although we were close then and follow one another on social media now, we haven’t made the requisite time investment to see one another recently.

This is what changes people in your life from acquaintances to friends and from friends to family: the amount of time you invest into your relationships. Your Significant Other becomes the most important person in your life, and eventually a member of your family (if you’re lucky) because the two of you invest time in building your relationship. Remember that “best friend” you had once upon a time? You spent all that time together and really connected because you spent that time together. There may have been some initial chemistry in any of these relationships, but the investment is really what will carry them through.

When people say “Relationships are work,” sometimes they mean that one has to make compromises and do things one doesn’t want to do, but more than that, on a basic level, relationships require investments of time and attention. What do you invest your time in, and in whom do you invest? Contemplating (and actually answering!) these sorts of questions are a path to happiness.

So: who and what?!