Why would you want to put yourself in a risky situation to save a teammate?

First of all, the game revolves around teamplay, and you never excel or play against better players, furthering your own ability to play, without teamplay. Never. I can personally vouch that a single player has never made it to the top without being a team player. If you can find a way to prove me wrong, I encourage you to.

Second of all. They are carrying you. Now I realize that everybody wants to be the carry. If someone on your team is doing an excellent job, you should probably do everything in your ability to assist them, including putting yourself in a risky situation. I will always return the favor. I'm even known for deliberately putting myself in a position I cannot escape from, just to drive the aggro away from my teammates. I do this because one death is better than three, and nine times out of ten, if they're going to tower dive anyway, i'm likely to the first guy they go for.

Third of all. It shows honor. Now I recognize that not a lot of people will show honor in a game such as this, but decent individuals will notice it, and return the favor. Them returning the favor, can likely change the outcome of a game. In fact I know it can. In an evenly balanced match, decisions such as defending your teammates at all costs is usually the decider of who wins the game. It's extremely difficult in most cases to rally your teammates into getting them to participate as a team. Being the first person to show initiative that you intend to die for them, albeit not feed illogically, does help with that difficulty.

Now of course you can choose not to do this, and find yourself in the stalemate division you can't get out of, but you think you deserve to. Or drop divisions, because if your opponents are competitive enough to utilize these techniques, that's exactly what's going to happen.