I believe some horses are perfectly happy by themselves. My stallion, Royal, is content whatever the situation is. After he bred (hand-bred) Lexus a year and a half ago, I had him turned out with her and he was such a gentleman; he would just groom her and play a bit. But he was also happy paying no attention to her at all during the whole day they were turned out. Once she had Briar, Royal was out by himself (and has been since), out of site of anyone except when he's in the barn, of course). He just takes things as they come. Where Lex and Briar will be visibly upset if they're left alone in the barn or paddock, Royal could care less; he doesn't even whinny when someone comes or goes. As long as there's food and shelter he is happy. Saying that, I think that he's definitely an exception and not the rule.