Why do people feel more comfortable in a relationship when the person has similar traits to their own?

Opposites attract rarely in people, always in magnets.

It could potentially be related to safety and security in so much as you’re more likely to predict (and accordingly act pre-emptively towards) the behaviour of a person who is significantly similar to yourself and thus may naturally feel more secure in their presence knowing that you possess a certain degree of control over them and the relationship in question.
In my experience when “opposites attract” (and their relationship is reasonably successful) though the two people involved in said relationship may have many evident differences there often remains a few key similarities that are sometimes not as obvious to the average bystander.

It is the birds of a feather flock together thing.

I feel comfortable with someone different. I do not want to date someone like me nor do I want to look at them and see me. Dating the opposite gives them and me a chance to share a different part of the world. It allows us to open each other to new experiences.

I have had relationships people with much in common and with little in common… I found compatability with them might have been different, but neither was better or worse than the other.

I don’t think it is that we want someone like ourselves as it is that we want someone who has shared core values.

My husband and I have very similar values and ethics, but we are very different on the surface. He loves politics. I hate them. We love to read different kinds of books. We have opposite taste in movies.

cause you can relate well to the person