I have a friend who is a sailor. Sometimes he takes companions with him, but frequently he sails alone.
He has time to think while he is out there in his boat on the water, and among the things he has thought about is the difference between solitude and loneliness. Solitude, he says, is when you are out on your boat and the wind fills the sails. Loneliness is when the wind dies. It is as good a description as I have ever heard.