In my adventures through silver and gold, I've noticed the BIGGEST cause of loss is people who don't know how to play the map. You'll get someone who contributes no pressure, doesn't rotate, doesn't roam, doesn't collapse, and doesn't know how to team fight. Just had a Malphite who went full AP and only ulted the enemy support Xerath. Guess why he thinks we lost? Not him! He did the most damage!
Oh but he never used his CC on the rest of the enemy team or the carrying YI.
This isn't Call of Duty, guys. Kills are nice but if you don't do anything with them or focus objectives, you're going to lose.