This is a great way to look at it. Repetitive meeting is definitely foundational to building relationships. Activities that involve some sort of accomplishment or triumph especially, like BJJ for you.

I didn't really put that together until now, but you're totally right. As a CS student, I've had group projects that involve regular meetings and a large project is developed over the course of a semester. Seeing my team mates constantly helps form bonds, but the wins and triumphs are probably the most influential.

I still remember submitting a project in the library at 2am and cheering that we were finally done. We got together for pizza and video games later that week for a more formal celebration.