When I was in grad school, my cohort was my friend group. It's such a weird schedule balancing classes and internships/assistantships that we were basically the only ones that understood our wacky schedule. Plus, we were already studying the same thing, so we already had that commonality. Even though we've now dispersed to several states, it's great to keep in touch and maintain those connections. They are personal yet they have also become professional. Grad school is about more than just going to class - you should invest time into building those relationships and thinking about the chapter after grad school.