This is a common sentiment, that your friends during formative years are those that are lasting relationships. I remember hearing a theory that correlated with my learnings: friendships are made stronger by having intense experiences. As a kid, you are constantly learning and doing things for extended periods with friends. You have way more chances to do something new or reckless or whatever. Similarly, many veterans are friends for life with their brothers in arms because, well, they were doing crazy intense stuff together for years.

As an adult, friendships are hard to grow for the same reason. Grabbing dinner with someone is not going to leave the same impression as, say, getting shot at. The adult friendships I have that are close I attribute to: 1. Friend's dad died shortly after we met, and we bonded throughout that time. 2. I went on an impromptu Vegas trip with friend 3. We went through grad school stress together 4. We spent a lot of time together at chess club. Nothing was intense, but frequently seeing this person solidified our relationship. Oh, he was teetering on divorce at one point, it could be that too.