There is a world of difference between solitude and loneliness, though the two terms are often used interchangeably.
From the outside, solitude and loneliness look a lot alike. Both are characterized by solitariness. But all resemblance ends at the surface.
Loneliness is a negative state, marked by a sense of isolation. One feels that something is missing. It is possible to be with people and still feel lonely—perhaps the most bitter form of loneliness.
Solitude is the state of being alone without being lonely. It is a positive and constructive state of engagement with oneself. Solitude is desirable, a state of being alone where you provide yourself wonderful and sufficient company.
Solitude is a time that can be used for reflection, inner searching or growth or enjoyment of some kind. Deep reading requires solitude, so does experiencing the beauty of nature. Thinking and creativity usually do too.
Solitude suggests peacefulness stemming from a state of inner richness. It is a means of enjoying the quiet and whatever it brings that is satisfying and from which we draw sustenance. It is something we cultivate. Solitude is refreshing; an opportunity to renew ourselves. In other words, it replenishes us.
Loneliness is harsh, punishment, a deficiency state, a state of discontent marked by a sense of estrangement, an awareness of excess aloneness.
*Solitude is something you choose. Loneliness is imposed on you by others.
We all need periods of solitude, although temperamentally we probably differ in the amount of solitude we need. Some solitude is essential; It gives us time to explore and know ourselves. It is the necessary counterpoint to intimacy, what allows us to have a self worthy of sharing. Solitude gives us a chance to regain perspective. It renews us for the challenges of life. It allows us to get (back) into the position of driving our own lives, rather than having them run by schedules and demands from without.
*Solitude restores body and mind. Loneliness depletes them.