Lots of really stupid arguments here, like me try to simplify this for some of you who understand literally nothing about game theory.
Each player mathematically represents 20% of a LoL team, not accounting for player skill, champion, runes, masteries & position/role.
MMR/elo is a system designed to give you a 50% win ratio after a reliable sample of games have been played (usually >25). It does this by finding the most similarly rated players (by MMR, not rank) and matching you with & against them. Winning increases your MMR, and the amount is dependent on a comparison between the average MMR of both teams. If you're the underdog, you gain more/lose less, if they are the underdogs, you gain less/lose more.
There are a couple ways to climb, but the main one is simply achieving a positive win ratio. If your win ratio is 51%, you will climb, but it will take a tremendous amount of games. You will gain the same amount of LP as you lose, and you will win 51/100 - meaning at the end of playing 100 games, you WILL gain LP/mmr since before game 1, but only a very small amount - perhaps around 18.
Your win ratio will only be 51% or higher if you contribute MORE than the average player at your current MMR. The better you are compared to the average player in your MMR, the higher your win rate will be. Don't use overall winrate - use OP.GG for past 20 games, since this is a much better sample, as it accounts for your current MMR.
Someone who 'deserves' a rating higher than their current one has a positive win ratio recently. They 'deserve' it, because all they need to do is continue playing at their own skill level, and they will climb to the MMR where they belong, then hit a plateau (win rates less than 60%). If you are platinum 2 with a 55%ish win ratio, that's essentially diamond 5 skill level. However, it takes a lot of games (and thus time) to get there, around 250 games mathematically. Those are NOT games to practice/gain skill - this example assumes the player stays at exactly the same skill level for all 250 games, without tilting.
It's understandable to be frustrated about teammates with garbage performance if you win ratio is around 55-60%, because you are better than the average person in your elo, but not to the extent that you climb incredibly quickly, and thus it makes you feel like you're being "held back". You're not actually held back, but it will take a lot of time (again, this has nothing to do with improvement).