Obviously, I havent had the life experience to really say much on this topic...
Its still an interesting and curious topic though, and I have some thoughts:
This question could be answered so differently depending on the friendships involved. My immediate thought was that, aside from chemistry, and physical shtuff, with a significant other they become as TE put it "like a body part". You take things they do and say so much more personaly. You feel like you should be able to depend on them no matter what. But then, as TE also pointed out, friendships can have that quality too sometimes...