What can I do?

It seems to me that every game I lose is because someone decides they want to lose the game on purpose. In my most recent game we were up by 6k at one point. We were winning team fights very decicevly and yet our nasus decided he didn't want to win any more. He kept running into the enemy team 1v5 just because. I am high/mid silver and this is just very frustrating. Thanks