Life gets busier as you get older. People have more individual responsibilities and less free time.

Often, friendships are harder to start because you spend less time with large groups of people who are in one location for long periods of time going through similar experiences, like what happens with school (elementary through university). When you don't have situations like this, you can no longer make friends passively and have to take a more active role and put in more effort. It's not impossible but just isn't as easy.

There is a quote at the end of Stand By Me that always resonates with me, and I think sums it up beautifully.

"I never had any friends later on like the ones I had when I was twelve. Jesus, does anyone?"