When people say they train their horse to smile - they're teaching the horse to do this on command. What this actually is is flehmen. They do it to elicit an instinctual/primal response to a food or smell. Sort of like how you associate a certain food to a feeling. It also can strengthen their strength of smell, although I've only heard that in passing and not sure if it's true. Look it up if you're more curious about the biological and psychological reactions. It's nothing bad, and quite funny when it happens, isn't it?
TLDR, he's trying to create connections, foals do it often.
Stay Calm - Ride a Friesian!