As most of you have seen, the horses have a happiness 'bar' that indicates how happy your horse is.
An unhappy horse will not perform too well in shows, so you would probably want your horses to be very happy.

Things that will make your horse happier:

1. Treats
Sugar => 1% happier, Cookies => 3%, Carrots => 5%, Apples => 7% and Grain => 10%
You can give your horse treats only once a month.

2. Feeding by stablehands
Since the stablehands are expert in feeding horses, the horses registered at the AVH Stable will be roughly 1% happier every game month.

The AVH Stable is also offering its expert services in treat preparations.
The offer comes with great discounts and you will actually pay less for these great services than you would pay if you bought the treats from the market.
For instance, you pay 1200 coins for one year of monthly carrot feeding.
If you bought carrots from the market every month, you would have to pay 2400 coins in total.

3. Grooming
You can schedule grooming for you horse in the AVH stable as well.
Basic Grooming => 3% happier, advanced grooming => 6%, expert grooming => 10%,