Why do horses put their head on a handler's head or over their shoulder?

I ride a horse that likes to do this.

Too many people push away horses that attempt this behavior. It's actually the closest a horse can get to putting his head and neck over your neck or back - as if you were another horse. Usually you'll see it when you approach a horse to pet it.

They're actually showing affection back and trying to "pet" you in return - it's rather like affectionate licking in dogs. (A dog that licks the hand that pets it is petting back).

So, it means they like you (but can be quite disconcerting!)