Maybe complicated is a better word. Or maybe an even more accurate way to put it is that people are complicated and can have a tendency to complicate relationships.
I would guess that everyone has experienced a friendship end without any understanding as to why. No falling out that you remember. No significant life change that would explain your drifting so far apart. No unforgivable wrong doing on anyone's part, but you sense what you would describe as "awkward tension" whenever you run into this person.
I have had two such friendships in my life. Ones that were deeply meaningful to me; that I'd expected would be forever friendships. The most recent one still really saddens me. We were the kind of friends who discussed everything and spoke the truth in love to one another always. Until, that is, it involved our relationship. The drifting seemed to take place slowly... I noticed it right away and missed our easy friendship but I let fear, hurt, pride and insecurity keep me from saying anything to her about it. She didn't say anything either. For a long time I hoped that we'd be able to adjust to our changing lives and move on with our friendship, but that never panned out.
Seeing her is difficult for me because I considered her to be like a sister and I have no idea what happened to change that. I wish I could have my friend back. It is said that friends are for a reason, a season or a lifetime. I kinda hate that poem. It accomplishes it's goal of poetically explaining why people come and go from our lives, but I really hate the going part. And I hate when you feel with all your heart that someone is a forever friend, only to find that perhaps they are not.