This may sound stupid, but...
When I was a kid, I had few or no friends. I had some people who tolerated hanging around with me for a short while, but none that I'd call a best friend. When people talk about friends who are like family, like say on Buffy, where her friends became her family, I always felt a sense of loss, because I've never had that. Because with me, people have never become close enough to become a part of my family, a part of my foundation.
I've always had this thing where I easily start conversations, but when it comes down to it, people don't stick with me, and that's only gotten worse as I've grown older.
As of now, I have no contact whatsoever with anyone from school, I don't really talk to anyone who isn't family, whom I knew before I turned twenty. I'll occasionally meet my brothers or sister's friends, but there's always a sense of removal. As for post that, I talk, very rarely, to some people I used to work with on a day to day basis, but even that's rare, and I always have the feeling that I'm forcing my presence on them. Same with people I used to talk to every day on IM. When I don't stay in regular contact with someone, I just lose touch. It's not because I want to, but because I'm too shy to restart communications, and I keep having this feeling that these people would no longer want to talk to me. And I have this dreadful fear of boring people.
(ps, anyone on my f-list whom I've lost touch with, believe me, it's not because I don't want to talk to you guys, but that I don't want to be the annoying clingy person that refuses to let go)
As such, family has become more important to me than anything. Because family is more than just friends, more than just lovers...
I think it's why I'm so against incest, and why I don't feel that a romantic relationship is so much 'more' than a family one. Because in my life so far, they aren't.
Family are the ones who will always be there, it's a bond, beyond anything, and if you lose that bond... it tears a piece out of you that you may never get back.
For me, I love the bond between Sam and Dean too much, to water it down, by making it something sexual. It's why even though I love Dean/Cas, I'll always see Dean's bond to Sam as more important. Because it's a core part of him.
Now family isn't about blood, as the Winchesters can tell you.
Take the family unit of Spike-Dawn,Buffy-Giles or Buffy-Xander. If you water those bonds down by making them romantic, then they lessen in importance, they become less than they are and they become replaceable. And I love those family connections too much to do that to them, or want to read about them that way.