To overuse the trite phrase, it's complicated.
I'm still working on myself (neverending quest) and fear disappointing/"failing" in a relationship with someone else. Fear of intimacy/distrust? Maybe some of that too. As a divorcee, I just feel really aromantic at this point.
And I won't lie, I like being alone, having complete control over my time, my home, etc. No arguing about where to eat, no feeling guilty for just spending the whole weekend in the mountains, no cluttering my home with knicknacks I don't care for. I keep the thermostat at 80 degrees Fahrenheit in Summer, and 55 in Winter (it never gets that cold, and I do use a space heater for just the room I'm in).