My wife and I are polar opposites in that regard. She can never seem to have enough friends, and they talk often. I think one big difference is how people are with nothingness. Because no matter how busy our day is, there's always periods of downtime...on a bus, on a walk, before bed, etc.
For example, I could sit in a chair and stare at nothing for a long time and not get bored. Mind always wondering, planning, thinking. She on the other hand gets bored and stir crazy in short order. And having people to reach out to chat with is a good cure for that.
I'm not suggesting one is better than the other, of course, just another difference in people.
All of that is to say, people like us who at least -say- we don't want more friends make it harder for people seeking, by removing ourselves from the pool so to speak. And I'd imagine between marriages, kids, work, etc... there's a lot of us like that.