Why does friendship so readily turn toxic?

Carlin Flora in Aeon:

Think of a time when you sat across from a friend and felt truly understood. Deeply known. Maybe you sensed how she was bringing out your ‘best self’, your cleverest observations and wittiest jokes. She encouraged you. She listened, articulated one of your patterns, and then gently suggested how you might shift it for the better. The two of you gossiped about your mutual friends, skipped between shared memories, and delved into cherished subjects in a seamlessly scripted exchange full of shorthand and punctuated with knowing expressions. Perhaps you felt a warm swell of admiration for her, and a simultaneous sense of pride in your similarity to her. You felt deep satisfaction to be valued by someone you held in such high regard: happy, nourished and energised through it all.

These are the friendships that fill our souls, and bolster and shape our identities and life paths. They have also been squeezed into social science labs enough times for us to know that they keep us mentally and physically healthy: good friends improve immunity, spark creativity, drop our bloodpressure, ward off dementia among the elderly, and even decrease our chances of dying at any given time. If you feel you can’t live without your friends, you’re not being melodramatic.

But even our easiest and richest friendships can be laced with tensions and conflicts, as are most human relationships. They can lose a bit of their magic and fail to regain it, or even fade out altogether for tragic reasons, or no reason at all. Then there are the not-so-easy friendships; increasingly difficult friendships; and bad, gut-wrenching, toxic friendships. The pleasures and benefits of good friends are abundant, but they come with a price. Friendship, looked at through a clear and wide lens, is far messier and more lopsided than it is often portrayed.

More here.