My theory is that we all start off as left-wing. You only need to listen to a toddler saying "it's not fair", this idea of fairness is the basic function of a young brain. We all instinctively want the world to be equal. Usually by early adulthood most think that those with more should share it with others, everyone should help everyone else. This is all lovely, it all sounds great.
But as we mature, most people appreciate that sharing only works if everyone pulls their weight. If everyone gets the same outcome whether you make an effort or not then there's no incentive to actually help anyone in any way. A significant part of the population soon works this out, and will quite happily let everyone else do all the work. This becomes corrosive - those working hard see that others who do nothing are as comfortable as they are, so they shouldn't bother trying either. The end result is that nobody cares about themselves or anyone else.
Leftyism is usually a phase of the developing brain, that most people work out sounds lovely but doesn't actually work in practice.
Those who are still in the leftyism phase of development think that those who think otherwise are selfish and/or actually want others to suffer. This just isn't true, those on the right want a better world but understand that apparent sharing and caring actually makes people worse off, not better.
But some spout the leftyism so loudly that even after they've already worked out it's a load of *******s they still feel the need to continue spouting as that's the tribe they've aligned themselves with, so they'd look stupid and probably fall out with friends if they stopped spouting it. It's a lot like a religion.