When your team is winning and you're in high spirits, don't get all nice and giggly in chat.
These same teammates (unless they're Challenger smurfs, part of the 0.000001%) are the same ones that will troll and screw you over in upcoming games.
Your teammates are your enemies. Until you adopt the "I'm playing a 1 vs 9 game" mentality, you will NEVER climb to high ELO (past Diamond III). Everything you've been told is a lie. It's from people who don't want you to climb. I'm the only one telling you the truth.