You're going to lose games in MOBAs. It just happens. You might have a great KDA; you may just lose the first team fight when respawn timers are high enough that the random group of 5 that has been raging at each other can shut up for 40 seconds to push your base. You may "lose" the game when a three man gank bottom goes bad because the enemy Sona just hit six when a minion died near her and she ults you to turn the gank around. Whatever, some of that is out of your control.
In almost every game you're losing, someone will say something like "The throws!" The primary cause behind "throwing" the game is that people get play over-aggressive and don't pick smart fights. I don't care if you're Leona/Draven bottom, that doesn't mean E in whenever someone steps in range of your spear. Let me tell you something; as a Taric or a Sona, I bait those spears to pull Leona out of position if Draven isn't ready to follow up (and in Bronze 1, Draven is never ready to follow up.) Then, we hurt Leona; sometimes kill her. Sometimes, my lane mate is the over aggressive one. I remember using my gattling cannon on Corki to clear a wave that had hit our tower when we were pushed, and Thresh took that as a sign to try and pull a full health Leona to the tower, who still had her W off cooldown. That trade hurt us enough we had to B or die.
Sometimes, I make bad engagements; when it is 4v4 in mid and someone over extends, I will initiate on them. I shouldn't; they may be baiting the initiate, or they may be legitimately out of position, but without letting my team know, they're not going to jump on them. Worse, if I'm locking down the enemy tank with Leona or Sej, my team might think it is time to burst down Singed.
What's the lesson from this? Aggression is dangerous, especially in low Elo/Leagues where a single mistake can start the snowball for the enemy team and your team is not mechanically good enough to overcome slight power differences.
Here's another place aggression works against your team: Chasing runners. If you have less than 50 percent health, and the enemy can burst for that, don't chase. I've seen so many teammates die when they chase Lux into the jungle because "We've almost got her!" Then, they eat lasers to the face. Let her run. Grab a tower, the dragon or just clear a minion wave. Don't chase people who can kite you, like Ez or Ashe. Never chase Singed. That does not end well.
Bottom lane is where bad aggression can snowball and destroy your team before team fighting even happens. Learn when your lane mate is poking and when it is time to go all in. If you don't have sight on their jungler and river ward is down, when Corki lobs a big one to chunk off some of Vayne's health, don't jump in. If Sona slides up in the bushes to power chord someone, she'll be back. She's not going to try and duel Ashe.
Pick your fights, don't chase when objectives are open (or if you are in the danger zone) and know when it is time to bug out. If you take an inhibitor, and the enemy team is up in less than 10 seconds, bug out. You may not get the Nexus turret in time, and they may counter push for the win if they ace you there. Let it go, secure buffs or just restore health and push a new lane.
My last point: Know what the enemy can do. Ezreal, Corki and Tristana (and to a lesser extent, Graves and Caitlyn) are not standing in your line of sight for no good reason. They all have good escapes; they are baiting you. When someone on your team pings you to be careful, it isn't because an ADC is dangerous and will blow you up. It is because you should assume the ADC is not alone at minute 32, unless you have enough wards up to verify that.
Here are some examples from the last few games, I've played where over-aggression threw the game in my favor:
1. I was jungle AD Yi, with BT, Phantom Dancer, Frozen Mallet (I find I need this as an AD Yi for when Highlander is down; plus the extra health lets me survive some AP burst to chop them down), etc. Our team engaged 4v5 while I was still on my way from knocking down a tower they told me to split push. The fight went badly; two of my team mates were limping away while the other two were death shopping, while all five of the enemy pushed to mop up, at varying degrees of health. I Highlandered then Alpha Striked out of the river bush on to their ADC, which jumped me to most of their team, killing the AP or the support. I popped Wuju Style steroids and proceeded to get a triple kill (the rest of their back line), which they could do nothing about what with all their CC being blown in the previous team fight and their tanks too far ahead to come back. My friends turned around as I Alpha Striked in for the ace on their front line bruisers. Then we pushed mid and blew up their Nexus. Just some wards in the river bushes, or simply saying: "Hey, that guy who can crit us for 500-600 nearly twice a second and burst all of us for around 300 is MIA; maybe we shouldn't over extend," would have saved them the game.
2. I was support tank Thresh. The teams were exchanging pokes in mid late game, but no one had any amazing pokes (no Nidalee spears or Varus/Caitlyn line attacks.) I went ahead and edged forward to grab some souls and to try and bait their melee heavy team to blow their gap closers so I could Q into their backline and force them to decide between saving their carries or engaging mine. Sure enough, they dove, then, I put them in the box. While my damage dealers lined up to shoot fish in a barrel, I Q'ed to the ADC and got in their face. I did end up dying though when I chased the fleeing ADC down to ensure the ace; but since I had heard the announcer lady announce a bunch of enemy deaths behind me, I knew there were no nasty surprises waiting, and it gave my team vision on where the ADC was so they could finish and push.
3. The last game changing one. I was Sona. Our split pusher was caught out and making a mad dash through our jungle, with the entire enemy team chasing. They had no vision, so as our pusher came through the bush I'd warded, I sat there with R waiting, and caught all five of them. The rest of my team's AoE then commenced. They never even made it through the bush to see that the rest of us were waiting there to jump them. If their support had done what I do, which is ward bushes ahead of where you're running, they could have avoided throwing the game right there.
Here are some less obvious examples, but still good lessons to learn:
1. I was jungling as Sejuani and pushing an enemy Xin off our Vlad; mid Jayce pings for help. I say I'm on my way, and after Vlad pools on the minions to start restoring his health while Xin is going B at the tower, I'm riding my boar to the lane. I'm not even near the bush when Jayce goes all in under the tower and dies. If he'd waited 5, 10 more seconds ,I could have ulted their mid and between the two of us, our burst would've been enough to get in, flash out. Patience is a virtue.
2. We had forced the enemy support out of lane, so I, as Leona, popped W and E'd onto the carry. Only, I didn't let my ADC know it was go time. I assumed when the enemy was at 60% health, we were at full, and it was 2-on-1 that meant it was time to go. I was wrong, and got killed surprisingly fast. Bad aggression, because I should have pinged to let them know.
Here's one that didn't win the game, but did net me as Corki a quadra kill.
1. We had just taken a tower, and the enemy were coming through the jungle. They had no vision, but our support had done their job. So, we saw them gearing up in the bushes. I went around the side and they charged at me. Then, I Valkyried over the trees as my bruisers came from behind. I moved into position while firing rockets, opened my gattling gun and we mowed down the entire enemy team. Sadly, I was the only one who thought we should start blowing up inhibitors. The enemy recovered and eventually pushed back to win. But, had the five of us pushed, that would've been game winning.