Horses are happiest when they have full freedom to graze at will, and when they are in a herd, however small. They evolved this way.

The Herd
Horses are herd animals, so a horse on its own is a very lonely horse. The social bonding goes a long way to making for a happy horse.
However, humans have found that it can be difficult to say, remove one horse from a two horse herd, when they want to spend time with the one horse. So we have tended to separate horses for our own ends. However, this creates more problems.
If you learn to understand the ways of horse, then you can remove these hurdles, AND keep your horse/s happy.

Grazing
Horses have evolved to spend 20 hours or so a day grazing or wandering around looking for food. To limit their ‘food’ time to a few minutes a day makes for two potentially very serious problems: great boredom and digestive problems:

• habits such as wind sucking break the boredom. A horse at grass will never have a thought to wind suck.
• a horse must be able to eat regularly, otherwise the digestive system starts to give problems

Besides the grass and hay, horses in work will need their diet supplemented.