Sociologist Gerald Mollenhorst conducted a now-famous study on friendship. In particular, he investigated how the context in which we meet people shapes our social network. One of his conclusions: We naturally lose about half of our friends every seven years. Given our ever-changing circumstances — we move, change jobs, get married — it’s logistically impossible to remain close to every friend we’ve ever had. My dear friend Debbie and I touched on this topic yesterday. Deb and I met when we were both pregnant nearly 39 years ago, and our enduring friendship is wrapped in layers of shared memories and experiences. We’re also lucky enough to have kept a few…