I've always had trouble "making friends." Adolescence was particularly confusing, as someone would be my "bff" one day, and my mortal enemy the next day, often for no particular reason I could see. I expected that this was just "childish behavior" and hoped that things would improve as I got older - I always sought the company of older kids and adults anyway. Alas, I am 37 and feel no closer to understanding "friendships" than I was back then, except to say that it seems to me that most people are "friends" because of convenience and proximity - they live near one another, or work together, or share a specific interest they can both participate in.
Worst of all is when I've thought someone was my friend, and would not harm me because I had not harmed them, only to find that they (like those adolescent girls) didn't really like me at all, and had been just faking friendliness and then sniping about me behind my back to other coworkers. I was so shocked (and frankly, frightened) by her two-facedness, I ended up leaving that job, unable to see how I could ever work with her, ever trust her, again.
I have this bad habit of assuming that if I mean people no harm, they'll mean me no harm. It seems logical, yet in real life it often spells disaster. In some ways I can be a very keen judge of character and seeing people for what they "really are," yet I still get blindsided from time to time.
For the most part, I'm generally quite happy with my own company, but sometimes I do feel lonely and isolated and alienated. Those times are hard.