I'm trying to come up with an answer that doesn't sound flippant, and I also don't want to just ignore you but the truth is I DONT KNOW! I have a...sense I guess you could call it. And I know which policies I identify as left wing. But what causes an individual to be left wing or right wing seems a lot more complicated. I think of myself as extremely right wing and present that way even though my policies are mostly in the middle, because I simply care about the stuff I am conservative about way, way more. I couldn't tell you why, because I don't know
But the most fundamental disagreement I've been able to reach when talking with social leftists is that they ultimately reject all non-voluntary identities and the obligations that come with. Ultimately the ideology is about maximizing choice, which is why it works to unravel social institutions that constrain behavior. I see social leftism as a force that chips away at weaker elements of a society. In a robust civilization this is fine, it only destroys the chaff, because there are what I guess you could call social antibodies that protect the important things that still matter. In an unhealthy society like ours it chips away at the very foundation (see how the United States is undergoing a mass conversion to a new self-loathing religion)
When I was a child, my peers were universally Christian. Every single one, except for one Muslim I knew. But their parents often did not take them to church, and they certainly did not prioritize religion. They barely saw themselves as Christians at all, it was just the received identity. So it's easy to see why so many of them didn't stay that way. That was my upbringing, and I only ever realized it when I compared it to my wife, who was raised as an ultra-traditionalist Catholic. For them, being Catholic was more than just a lukewarm identity, it was carved into almost everything else they did. Which is why, of her many family members, vanishingly few have left the church. This is the difference between a real identity and a facade, and it's the difference between a sturdy and ancient tree that requires hours of toil to cut down and a rotten one that can simply be pushed over. Part of the reason social liberalism is so ascendant, in my view, is that the rapid technological changes of the past hundred years or so has forced so much necessary change that it's incredibly difficult to discern what traditions are good and necessary and which ones can be rightfully left behind. If any of that makes sense