The game is usually decided by how cohesive your team is, not KDA. If your team can't properly engage, doesn't ward, doesn't focus the same champion, and can't get objectives then it doesn't matter how many kills you get or how well you did in lane. You will lose. I've had this happen most of the time, and a lot of times the deaths start rolling in after those failed team fights.
I think the thing I see a lot in solo queue is someone acting like the team captain and making such bad calls by underestimating power spikes and gold differences. If most lose lane or an enemy is more powerful than you, you should not be team fighting. Your opponents are stronger. However so many people want to team fight when they're behind, and it exacerbates the problem. Another big thing is knowing when you can contest an objective. If you can't because you're behind or minions are at your base with only a support defending, then don't try it. Fall back, ward, and defend. Don't lose half your team, or worse, get aced.
If you see someone struggling on your team there are a few things you can do. First you need to be polite. Being snotty is going to make the problem ten times worse. Let them know it's okay to lose a tower, and encourage them to build defensively/ward. Tell them that you will be in their lane to help as soon as you're done with whatever you're doing. Just treat others how you would want to be treated if the situation were reversed.
Too many times I've seen people spaz out on someone who got pressured so hard in lane with no help from teammates. Be it the enemy jungler camping top lane, or the support leaving the bot lane way too early leaving it 2v1. If you're the jungler and you're having issues, let your teammates know you were counter jungled and you're going to be weak for a little bit. It's sad to see so many people capable of making a comeback lose faith because a teammate is being obnoxious in chat about their mistakes like they don't make any themselves.
