Mares LOVE being called "Mommy" by their potential mates, so much that they immediately become aroused.
It's an instinctual response, playing on both their procreation and maternal instincts, as her stallion or man simutaneously acknowledges the mare as his superior (and thus less chance for him to abandon the foal), is being cutesy, clingy and extremely submissive (and rideable); pushing the buttons on the mare's maternal instinct, and is viewing the mare as a potentional sexual partner, wanting to make her a Mommy for real.