There are a few reasons why horses lift their upper lip. The most common reason is to check for the presence of food. When a horse lifts its upper lip, it is able to smell better and can determine if there is anything edible nearby.
Another reason horses lift their upper lip is to express their emotions. When a horse is happy, it will often lift its upper lip in what is known as a Flehmen response. This is when the horse curls back its lip to expose its front teeth and gums. This is often seen when horses are smelling each other or when they are interested in a new smell.
The third reason horses lift their upper lip is to communication with other horses. When a horse lifts its upper lip, it is often trying to send a message to another horse. For example, a mother horse may lift her upper lip to show her foal that she is nearby and is there to protect him.