I can remember being younger (much younger) waiting for friends to come and pick me up for social events, with the fear of being left behind, or left out...and sometimes I was - as a much older adult (closer to 50 than I am to 40) it is depression that keeps me away from people. I lack confidence, my self-esteem is shot, and I am afraid that I'll end up dying, alone and afraid...

The reason I think that this fear is very common is that humans are social creatures. I'm not sure what you're dealing with - as you can gather, I'm dealing with depression. As such, I'm very good at putting myself down, so much so that I sometimes don't feel worthy of relationships. That, in and of itself, pushes people away...just my two cents...