The ability to be alone with yourself
This means that you enjoy people but also enjoy your own company. That you regard yourself as a human being and worthwhile company is an indication of much health. Not all people do. Some are terrified to be alone because alone is really alone--to them, it means being with nobody at all--and as such is an indication of very low self-esteem.
The ability to be alone with yourself and to be self-energizing , self-stimulating and self-igniting--the ability to be either active or passive when alone but to know freedom from boredom and anxiety--at once makes you independent, self-reliant and thus a more mature, as well as an easier person with whom to have a relationship.
* From the book, The Winner's Notebook by Theodore Isaac Rubin, M.D.