Ranked Games

I finished my first season provisionals last night. I only won 2 of my 10 games and somehow got placed in bronze 1 (no complaints here I should be able to get silver no problem as I was playing silver players in my provisionals). Anyway overall I didn't play well, but I didn't play horribly either. I had a lot of pretty average play. I didn't feed (except for 1 of the games I had to play support) and I didn't get fed (except for the 2 games I won). So overall pretty average. In the first game I won I was playing zed and not only did I do well but the rest of my team did well also (it was a slaughter). In the second game I won I was playing sivir and I did really well (24/2/8 or something like that) and the rest of my team played a pretty average game (didn't feed or really get fed at least not until later). However, in the games I lost no matter how I personally did the rest of my team except for a few people here or there did absolutely terrible. I'm not going to complain because those kinds of losing streaks happen (it happened to me on zed not to long ago, I would do really well and the rest of my team would do really poorly), and I did get bronze 1 so like I said I shouldn't have trouble getting silver. Anyway I was wondering what causes you to lose the most of your ranked games? For me so far it has just been bad teammates.