When a dog lays down and intentionally puts part of its body in contact with you it's an indication that dog thinks you're a special person. Maybe you're the dog's alpha or someone the dog thinks will protect it from danger. It's an act of bonding and is something we dog owners invite and value in their dogs.
Dogs are often very subtle about doing this. Sometimes you don't even realize it until you or your dog have changed positions.
If it still annoys you, get a goldfish.