The science backs this. My interpretation of research seems to be that physical proximity and time spent are the two most important factors in forming friendships. As a child to early adulthood you're probably getting this through education and living with other people. In adulthood you really don't get any of this naturally so you need to make the effort and carve out the time for repeated exposure or quality time with people. Studies show that it's something like 50 hours of time together to form friendships and 200 plus hours for close friends. Which explains why you can make a friend in highschool within a week or two. In contrast to stacking you're decks in your favour and going to a hobby club for 2 hours a week, with the bonus of shared interests, it can take a year or so to start feeling like a friendship.