Actually, folks, I have to disagree to the degree.
The more communications with other people horses will have - the more desensitize they'll be to people and won't freak out in situation when someone besides the owner should handle them for the farrier, vet, or (especially) in emergency. Of course I mean people, who know how to behave with the horse and how to manage one. And riding is the whole different story of course (there are very few people I'd trust to ride my mares), I'm talking about handling on ground.
If you are jealous that your horses will love other people - I do NOT think it'll happen if you are the one who messes with them on daily basis, rides them, and may be feeding them (and the last one not even the most important in the relation).