[a008t]

When you are in full-time education (school, university) friendships come naturally from the people you share the experience with. Work doesn't have the same effect because being professional usually implies not combining work with fun and not getting personally close to people.

So when you are in your 30s, you have limited time and limited opportunities to make friends. If you don't spend a lot of time hanging out with someone it is tough to make really good connections. Other factors come into play as well - comparing yourself to others, more niche interests, lack of desire to invest time, having a partner, etc.