Loneliness™ is something you can feel even when you're surrounded by people, who're good to you and treating you nicely etc.
On the surface it can seem similar to Depression: "What do you have to be so sad about??"
It can be similar internally too: a vague sense of not-quite-doom, or just an invisible wall in your ability to feel "happy"
That all the people who hang out with you and laugh with you, are not really your friends if push ever comes to shove, that you're going to be alone in the things you care about, even if you have a loving partner, etc.
Or maybe just even simply feeling out of place in the overall culture you live in..