A really good friendship, yes. That ruined me, no. I’d say the opposite.
I had a best friend in college. The first year we didn’t take to each other, but as sophomores we became close when some upsets brought us together. We remained close for many years even though we lived far apart after college. There was nothing we couldn’t tell each other, and although we were very different personalities we had a level of understanding that I’ve never equaled with anyone else.
There’s never been anyone else in my life with whom I felt so completely at ease and whole. I’ll never regret learning that that’s possible. I treasure the memory of it.
And maybe it was just the right time for it: the shared stress of college life, the intimacy of a dormitory setting, the relative maturity of late teens combine with the relatively short life history, short enough that it was still possible to convey to someone else most of what had shaped us to that point. By the late twenties or thirties, a person has accumulated so much personal history that no one else is really ever going to get who we are. At least, that’s what I think.
But far from ruining me for other relationships, I feel fortunate to have had one such in my life. It’s not something everyone gets to have, and I don’t expect to have it again. In my case, it takes me a very long time to establish trust in a relationship, and some people just can’t wait for me to get there.
Belatedly, I’ve realized that that’s one very good reason to stay in touch with siblings: that shared history that you can never explain to others. As Edith said to Mary in one of the last episodes of Downton Abbey, “Someday we’ll be the only ones left who remember all this.” With our parents gone and no “back home” to go to, we’re already there.
Romantic relationships, of course, encompass a dimension that was absent in my relationship with my friend, and they have their own wonderfulness. Here I’m just responding to the question about friendship.