It's called co-founder bonding and it isn't talked about enough in the startup community. You will see most of those things are not business related at all. It's really the best if your co-founder becomes simply a close friend. Of course, it's not always possible to make him one of your best friends, because co-founder should be complementary, so they should fill the areas of expertise that the other co-founders doesn't have. So, by definition, your co-founder should be the exact opposite of you. In theory, this would make things a bit, in reality, even though co-founders complement each other, they are not so different after all.

Let's start with the non-business related things that strengthen the bond with your co-founders:

1. Take him for a week-end hike with your friends
2. Go rafting
3. Have your kids and his play with one another
4. Grab a beer with him
5. Learn his humor and adapt to his personality.

A few business related things

1. Get him to do favors for you business-wise and non-business wise. It will make him like you more. (https://www.forbes.com/sites/sap/2011/11/16/do-me-a-favor-so-youll-like-me-the-reverse-psychology-of-likeability/#6e63dff574a5)
2. Make sure he has enough equity to be emotionally invested in the company. You can have a co-founder that has 5% in the company and he might stick around for 2 years, but if things are going slightly wrong or it's going too slow, he will quit. A co-founder with 40% equity has a much higher pain-limit obviously. So, you need to make a trade-off about equity you keep and pain-limit of your co-founder.
3. Make sure your vision is somewhat aligned with your founders vision. Your visions won't ever be exactly the same, so you need to make a compromise. Make a step towards him isn't a bad idea if the alternative is him leaving the company after a few more months.
4. Have difficult conversations about the business with him. I have seen co-founders that have several things that really annoy them about each other, but they never mention it in order to not disturb the peace. This might work short-term, but long term, you will create a weird atmosphere, where you have to be artificially friendly and can't talk about the things that should really be addressed.