I can relate, I could have written this article a decade ago. I was always the single person and I would lose friends who would pick me up and drop me off depending on their love life. This is why I created such a full life for myself, as I could not wait around for a friend to want to see me.
Now, I feel I lose friends when they have children. I do not want children and it can be very difficult - as a female - to stay connected with friends when the new label of "mother" comes into play. I am happy to make 75% of the effort, but I believe that friendships must be reciprocal, and I need SOMETHING back, this is not asking too much.