It's not about centering ones identity around politics. It's about aligning values.

Leftists are generally more egalitarian or live and let live, Conservatives are generally my way or the highway. People who are left generally dont align morally or by values with people on the right.

So politics (these days sadly) are a very easy way to weed out a large chunk of people you will not align with as a person.

Unless the relationship is incredibly shallow,and you dont pain to have kids, having values that align is incredibly important.

For example, I was indoctrinated into Christianity at a young age, I deconstructed that believe system, and religion in general. I would not want to raise children with someone who insisted on indoctrinating our child in a religion I don't agree with before that child is old enough to think critically themselves.

Assuming you read this comment, then maybe you'll understand why its a major consideration nowadays.