This probably isn't true of everybody, and I certainly don't mean to imply it should be, but in my opinion there's two types of relationships.
In one type of relationship, one person wants the other to be their Platform. Like their own private theatre with a projected spotlight and built in audience, it's their place to feel special. The other person must make you feel special, make you feel important, make you feel desired, empowered, needed, precious, important. Personally, I think this relationship doesn't work; it's foundation is insecurity.
The other type of relationship, one person is the other person's Home. A place where they can close the door, lock out the world and just...be as they want/are. They don't need anyone to make them feel special, or to empower them or (frankly) to make them feel anything other than welcome and accepted. Someone with which they can untangle their own thoughts and doubts, work on their weaknesses, open wounds and heal hurts, revel in their achievements and enjoy their passions.
As I've gotten older, I've realized too late that real romance isn't about theatrics and spectacle, but rather it's a hindsight moment, like a warm breeze that passes you by; you either notice or you don't and either way, it doesn't matter. I've been in relationships that are demanding, that want you to be better, want you to be different, want you to be yourself. And I kind of fell into that social construct, putting up with it because...well, that's the way things are. You do that, you have to. As a guy, I've had it ingrained in me that women are over-emotional and kind of unstable and that's why we love them and that's just how it is. And as I got older, it just doesn't work for me.
Instead, I found someone who didn't need me to be anything but happy. And respectful...but, when you're with someone who is just genuinely happy in you being happy (and vice versa) then that part is easy. I was always taught women were a certain kind of way but I've learned that isn't true; a lot of women are whether that's the fault of them, men who drive that behaviour, society or all three, I don't know. But I found someone who is just emotionally mature. She doesn't need anything from me, and she doesn't need me. She wants me. And there's a world of difference there.
Sorry for the long rant. It's late and it's been...a reflective few days. But what's a common mistake when choosing a SO? Well, for me it was accepting what everyone else accepted; choosing by other's values. A part of me wishes I had learned this lesson earlier, but...a part of me is glad I didn't. Who I found now makes every broken heart it took to get to her worth it :)