I've co-written two movies. Each one took three years. They should not have taken nearly that long, and the lack of accountability probably ruined our chance at being rich, famous, and eventually disgraced by some kind of scandal. So sad. Later, I worked with a guy who had no kids and a boring job. He wanted to make time to write with me, even if we mostly drank and talked about the good ol days. But we both worked on our own in preparation for the meetings, and we consistently met, even if it was once every two months. Find someone who is at least as much a loser as you. Someone who doesn't have six kids and a nagging spouse. Kids and spouses are real-life creativity vampires. Like my kids: they want me to help edit their Minecraft videos when I have a fantasy novel to work on.
Likeability
At first, you won't think this matters. I worked with a guy in a film Meetup who was wonderfully knowledgeable, and I really thought we'd be able to work together to create some highbrow content. Then I wrote a snappy little script, which I had to change because he suddenly didn't like my initial idea. And then he tore it apart, saying the main character, who was a woman, should not be talking about men with the other woman (even though I did it on purpose in order to critique the stereotype). He also told me it was racist to have the Indian (from India) character speak with an Indian accent, even though his favorite TV show (The Simpsons) did exactly that. And the Black character could not use slang that I knew to be accurate. And it went on and on until I asked him how anyone is supposed to write any dialogue in his world. One or two more explosions, and we were done working together, his knowledge wasted, and probably not for the first time.
Ability
I've worked with a few people over the years that treated writing somewhat how some clients treat my website design. They think me showing up will just make the content happen. I've heard cool ghost stories and dramatic life stories. But I can make those up on my own without a co-writer, so I have to be sure the person I'm working with has more than just a couple of ideas. You might feel high and mighty to be more talented than your partner, but you'll eventually wonder why you had a partner in the first place. Sure, you can have differing roles, and those can even change as you write, but if one person is simply working harder or that much more talented, move on.
I'm sure there's more to consider when finding a partner for writing. However, the three categories above should help you find someone. Speaking of which, I'm looking for collaboration in the Jacksonville, FL, area. I'm good, maybe as good at editing as writing. I spent 12 years teaching writing, and I often know how to help others better than myself. Anything we'd write together would be 50/50, but I'm willing to help for free, as long as it's a give an take. I've also been writing song lyrics (I have one musician), so a local musician who wants to record original songs might be a good fit. Or something entirely new and different for me. I need someone to help create songs for my musical play, too. Let me know.