What are you hoping for?

As a technically-minded person, I'll just come out and say that I've only ever met one founder that was my friend, and they were a deeply technical person. I don't have anything in common with MBA graduates or post-management types that spend their life reading Medium posts, browsing Linkdin or tastemaking on X. I'm willing to work with these people, but asking to be my friend is a dead end.

I'm not saying that your cofounder is a carbon-copy of me, necessarily. But it sounds like you're conflating personal needs (eg. "I need friends") with your workplace situation ("my cofounder is not my friend"). You could rectify this by expanding your personal network and letting your technical cofounder focus on the technical stuff.

There's no harm in the relationship being strictly related to work, especially if that's what brought you two together in the first place and you found that you complement each other well. You help each other out, and you're able to have constructive debate, particularly if you both have different views on stuff or different outlooks. Those are all really healthy things.

If you want to try to build more of a personal connection, it's more about introducing small talk and banter into whatever frequency you guys meet (stand-ups, etc.). Learn a little bit about each other and what the other person's interested in. You may find common ground that you didn't know existed. For example, my co-founder and I both learned that we both really enjoy cooking.

At the end of the day, as long as it's strictly professional and focused on work, there's no harm in it. As long as you guys enjoy working together, are driven toward a common goal, and get joy out of the time that you do spend together, even if you're not going out and doing things together when you're not working.

Nothing you said is a red flag. Introverts work with extroverts all the time.

Co-founder relationships are like marriage times two. You spend all day together, you’re financially bound, you will disagree and you will need to resolve conflicts. You need to measure that.

Deliberately bring up the thing you disagree about most. Work together till one convinces the other. If that process leaves a bad taste in your mouth, break up early. If you can’t do that with anyone, you need some emotional intelligence before you found a company.

I had a co-founder who I really liked. We had a disagreement where I was seeking a fair outcome. He threatened to kill me and left the call. This is an extreme case but it illustrates the need to measure conflict resolution.