Horse by nature are free creatures. They roam around and graze to different pastures. They have always been mystical creatures and are very aware of their surroundings. That's why horses that are put on barn tend to get agitated easily.
Agitation leads to a lot of sickness too. Research shows that barned horses develop colic on 80% of their lifetime. They get stress and in turn produces more stomach acids. That's why its important to invest on equine ulcer medication to counter-effect the horses stress. Another thing to invest in is horse probiotics. I have found that it benefits a lot to the horse in the long run.