What is loneliness?

I was reading the answers in this question, and it got me thinking about how people at work say they are lonely certain times, and how my therapist keeps pestering me about it…...
I’ve googled it and got the definition of it but I still don’t understand it.
I don’t think I’ve ever experienced this…

I live alone, I have a few friends I see every couple of months, I don’t usually talk to the people in my apartment building, I never talk to coworkers outside of work

but the best part of my day is getting home and no one is there, I turn my cell off, and love every minute of it.

any help with this is appreciated.