Horses are psychologically programmed to seek out the company of other horses. As they instinctively know there is safety in numbers, they feel most comfortable living in herds. In the wild this protects each horse in the herd from being attacked, injured and/or killed by a prey animal.

This is a very strong instinct that most domesticated horses still retain very strongly. No matter how special the bond is between you and your horse, he will still enjoy and seek out the company of other horses if given the opportunity. Horses are very social animals who really like the company of other herd mates even if they don't have a smooth relationship.

A horse that is used to being around other horses will show definite signs of anxiety and stress usually after only a short period of separation. Anxiety can be played out by ignoring commands, nipping, walking away when given the command to stay and other acting out. Much like an unhappy child will act out displeasure by misbehaving,your horse will sometimes also use this method to show you that he's unhappy being separated from the company of his kind.

Do not punish him for being anxious, instead respond to your horse by using a gentle voice and affection. He needs to know he can trust you and once he is calm you can go riding, groom him or do whatever you intended to do. Meet any anxious outbursts with kindness, you will find they will greatly lessen with this method.