So, DP and I actually knew each other for a couple of years before we got together. A good friend of mine in uni worked in a shop and they'd all have nights out and I'd end up bumping into them or tagging along. DP and I really had nothing in common, but we'd always end up chatting to each other for a decent amount of time. Nothing romantic or sexual about it at that point, I just felt really comfortable in her company.
We were never friends. I never saw her outside of those nights out, never thought we should hang out for a pint one night or anything like that.
Uni finished, I went travelling for a year, came back and suddenly we fancied each other, were both single and the cheap beers in Jumpin' Jaks did their job one night.
I'm not really a people person. Fine for a day or so, but after that I've generally had enough of whoever I'm with. Even my two best friends, who I love dearly. We get on great, have a laugh, talk about all our shared interests and then go home and I think "Nope, don't need to see them again for a week". The few times we've spent a full weekend together I'm generally crawling the walls by the time we're done.
I've never ever felt like that with DP (or most of my previous girlfriends). Yes, sometimes I want a couple of hours to myself, but I've never felt like I don't want to see her for X amount of time. She feels like home, even in those drunken conversations before we ever got together. I feel comfortable with her, I feel I'm more "me" with her than at any other time. And when we first started dating, it didn't matter that we didn't really share any interests. We just never ran out of things to talk about, stuff that we'd never shared with anyone else. Things that would bore me coming out of anyone elses mouth. I learnt more than I ever wanted to know about the Kardashians by about date 5, and somehow it wasn't boring to me, because I was learning about her. 20 years later we've still not run out of things to talk about.
I think thats the difference for me, once you take out the feelings and sex stuff. My friends are the people I want to share the things I love with. My partner is the person I want to share everything with.
I'm always going to enjoy discussing the finer points of Star Trek with my friends, or bitching about the place we work, because we have that shared experience and knowledge. But DP is where I'm going to be happiest, even if she can't tell the difference between a Klingon and a Romulan.