I spent my youth losing at video games. Not that I was worse at them than the other kids, but video games at the time were designed to make you lose all the time. They evolved from arcade games, where it was imperative that the player lost after some time and had to insert another quarter. It took nearly 20 years for the first person to "win" at Pac Man. Only later did game designers come to the surprising conclusion that most people prefer winning over losing, and these days most games offer at least some guaranteed win method, either in the form of an "easy" difficulty level, or built-in cheat modes.
Expecting to win has changed the attitudes of players, especially those who haven't experienced the "you always lose" era. People now quit games when they don't win. Even games which technically can't be won, like MMORPGs, these days carefully avoid to give anybody the impression that they just lost, especially to new players. Now I'm all for optional easy modes, but with MMORPGs usually offering only one difficulty level for most content this has led to abominations like the current World of Warcraft new player experience, where it has become actually extremely difficult to die. Making it possible for everybody or nearly everybody to win is a good idea. Making it impossible to lose isn't, as it takes all illusion of danger out of a game in which you are supposed to play a hero.
But by far the worst effect of this expectation to always win has been on PvP games. I've seen numerous forum posts from players of World of Tanks complaining that they win only half of their games. It is mathematically rather obvious that everybody winning half of their games is actually the theoretical optimum, the best you can reach, the perfect matchmaking. Any league or ladder system strives to reach exactly that. If any player wins more than half of his games, then another player necessarily has to lose more than half. If already losing half of your games is sufficient reason for a player for a "I quit" forum post, then what are the chances that the players losing more often than not will stick with a game? Games where some players consistently win more often and other consistently lose more often quickly move into a downward spiral in which the losers quit, making another group of players the new losers, until those too quit, until nobody is left.
Winning is not the purpose of playing. It certainly is an objective in any game with a win condition. But the success of games that can't be won shows that winning isn't strictly necessary to have fun. It is the playing itself which is the purpose of a game, not the winning.
A win to loss ratio of 50:50 is not only the best result any matchmaking algorithm can achieve. In games which have at least some influence of skill and aren't purely random, it also is the 50:50 win to loss ratio of a team at which the individual contribution of the single player is most likely to affect the outcome of match. If the match is unbalanced and near certain to be a win for one side, neither the players on the winning side nor on the losing side can actually do much about the outcome. If the outcome is on the knife's edge, a single outstanding action can cause the result to tip in one direction or the other. But as this outstanding action is equally likely to happen on either team, that doesn't change the overall 50:50 ratio.
Most players do not understand that. They believe that they are superior to the rest of the players, and that the only possible explanation for them not winning all the time is that skill isn't a factor at all, and that the result is purely random. But playing World of Tanks for a while you quickly realize that the team victory comes from the sum of the performances of the whole team. Luck and random chance plays a role, but an observant player always knows at the end of a match whether he personally did good or not. And the reward system helps by putting a larger emphasis on individual contribution than on team victory. You gain more xp by doing personally well and losing the match (as frustrating that might sometimes be), than by doing little and being part of a winning team. Furthermore the reward system is positive sum, so even a string of losses only results in you advancing slower. That is a lot less frustrating than a zero sum or negative sum PvP system where the loser is actually worse off after a battle.
By de-emphasizing the importance of winning, we get back to the true purpose of playing, the playing itself and having fun. A good match is one where you had fun, regardless whether you won or lost. If you manage to mentally disconnect having fun from winning, you can get a fun:unfun ratio of much better than 50:50, in spite of the perfectly balanced win:loss ratio.