I've been studying psychology and friendship for a few years now and I believe this is the best formula for friendship I've come across, I thought I'd share it.
Time spent x Vulnerability = Depth of friendship
It takes 200 hours spent together to have an actual best friend, but if you spend 200 hours with someone with no vulnerability (say a coworker) then the friendship will still remain shallow. So for those who have problems with being vulnerable (like me), It's good to keep in mind the importance of opening up to form deeper bonds