It's a rare horse that is truly content to be alone; though they can learn to do it, it can be one of the hardest lessons of a horse's life. Some people start keeping horses separated, starting at an early age; though these animals may be easier for their handlers to work with without the "herd sour/buddy sour" behavior, they can be so socially inept that they don't know how to behave when they are in the company of other horses.
That makes sense and sucks for the poor horse :/