Speaking from experience, I cannot tell you the profound joy of working with horses- and I know that joy is mutual. I cannot prove it to you, or anyone else for that matter, unless you experience it for yourself. So it's really just words.
Horses though are like dogs. They are both far removed from their wild ancestors. Like dogs, horses have been bred and developed to be in close contact with people, to the point that they need us. Not only for their food, water and shelter, but for mental and physical stimuli.
A riding horse is meant to be ridden. Like a hunting dog left in a kennel, a horse without a job is going to go crazy. They get fat, lazy and obstinate. Unsoundness and mental problems inevitably crop up. The horse will not, and indeed cannot, be happy without fulfilling his need and purpose to, for and with humans.
That said, you can see in any working horse a quiet happiness that does not exist in a horse that is simply left to his own devices.