What do you do with a negative coworker?

There’s a person I work with (and technically work under), who is constantly finding something to complain or gripe about. I’ve tried listening, seeking out solutions, and at times encouraging them to look at the positive side of things.

But they are just so negative! To the point where I feel like I’m starting to follow a pattern of negativity! But I don’t want to be this way and I’m choosing to be more solution-based rather than problem-based.

What should I do?