If you think about it, ideally, everybody should have around a 1:1 ratio of wins to losses. When you win a lot, and you have shown outstanding performance in your game, your ELO increases, but then, it also senses that the people you are being matched with are too "easy" for you to play against. Players of higher ELO who have the same or better level of playing as you will be in your "pool". However, if you lose a lot and have been suffering in your games, your ELO decreases but the system also senses that the people you play against are of too high of a rank and you will not do well. This causes the system to shift you into a lower "pool" of players when you are in matchmaking queue to compensate.
TL;DR LoL wants everyone to have a 1:1 win/loss ratio and perform adequately ingame with others that are on the same skill level as you.
I hope this answers your question.