I agree with Trevor to some extent.
However I don't see loneliness as being unable to spend time alone. I can do just fine spending lots of time alone.
I think "loneliness" as a word comes up when you feel the need, in a given moment, to share things with someone....but you look around and noone is there to share it with. Or there is a particular life problem that you would like some social support with and you can't see anyone to talk to about it.
I don't look to other people to complete me, because fundamentally my happiness is my sole responsibility and my "being OK" is a state of mind. However, being social animals we do need normal social contact or we feel there is a gap somewhere - and that is loneliness.
Sirona