I've always found that being in a serious/stable relationship tends to slowly cut people off over time. I honestly like being single in my mid-30's because there's always a little kick in the pants to get out and socialize that my married friends don't have.

Every time I end up in a stable live-in relationship (and I've seen this happen to a lot of my friends as well), the idea of going out and seeing/making friends more than once a week feels more and more unnecessary over time. It's so much easier to cook dinner and watch Netflix rather than get dressed and be on the prowl (both for romantic relationships and interesting people to hang out with).