You meet them one day; a chance encounter. But you both have lots of friends each, so you don't talk very often. Another chance event brings you closer. You feel like you're becoming good friends with this person. Now they're your best friend. What about your other friends? Too busy for you, annoying, inconsiderate, rude, back-stabbing. Name by name, both of your friend groups shrink over the course of several months. 

It's just you and them. I guess you only really need one friend. They're sweet, patient, kind, and considerate. They listen when you talk, and you listen in turn. They're the best person you've ever met, and that will never change. 

You call yourselves Time and Space: such mutual understanding, awareness of each other's needs and feelings. Perhaps Nothing can separate you.

Their personality, their tastes, interests. You pick them up. You learn about paganism, art, marine life, all their favourite things. You show them what you learned, and they love you even more for it. *This* must be who you are; you are like them! They like it when you do these things, and that makes you feel almost real, like someone that actually exists.

You stay together. Months pass. Then a year. Something changes, fades. Although you may have convinced yourself otherwise, you don't really have any interest in the things they like. You have no urge to know any more than you do. You forget the pagan ceremonies and the manta ray facts. You forget who you are again. 

You feel so sorry for them, so guilty, because they think you devolved. They miss the old you. They miss the romance and the cuteness. But you can't bring yourself lie. You feel empty and soulless and could never fake those feelings that you had before, as that would be cruel and dishonest. It's not their fault, it has to be yours. But you can't fix yourself, you don't know how.

They find someone new. It's only a matter of time before they disappear. They seem happy together. But you feel jealous. Like a wounded animal, you lash out, become verbally abusive, childish, and picky. You describe every flaw of this new partner, as if that would recover the relationship from the rot.

Then, it finally happens: they've had enough of you. The door is slammed in your face, and that marks the day of freedom. The last threads of stability snap, and what shambled together personality you once had falls apart like a split garbage bag. What remains is a loose collection of thoughts in a vessel, like a ghost with unfinished business.

You wait. Weeks pass. Then probably ten months or something, you don't actually know. Maybe you just hallucinated the entire relationship. You can't decide if you exist or not. But one thing is for sure: you could never go back into another relationship. You're glad they're gone, and you hope for their sake that they don't come back, no matter how much you can't stop thinking about them.

TL:DR: Being in a relationship gives you perspective about who exactly you are.