That surprises me because I don't often feel lonely. I may feel bored or out-of-place or weird, but not lonely. I don't feel separate from other people because I know I have lots in common with them, even if we're not actually face to face or talking to each other.
Even when I lived on my own, before I met Jenny, I didn't feel lonely because I got out and about and didn't feel a need for other people's company as I had plenty of company at work.
Not that having company is necessarily the answer to loneliness. I can be in a crowd of people and still feel lonely because I don't feel they understand me or that they're on my wavelength or they share my interests.
The feeling of loneliness has many causes, in particular not being happy with your own self and wanting other people's reassurance or support or appreciation. I suppose it also includes the feeling of missing out, that other people are having a better time than you are. I guess every lonely person has their own definition of loneliness and what it feels like.
It doesn't help that the prevailing view of human beings is that we're social creatures and need other people around us to feel good. That ain't necessarily so.