Why do llamas spit?

"Llamas, like most members of the camelid family, sometimes spit saliva. This behavior may be done to show dominance towards other herd members, for fighting, or for defense."

About two years ago, we went on a field trip to a zoo that had a very friendly llama.  However, after too many hugs that day, she had had just about enough.  Trevor had walked by her a little too close for her so she turned and spit on him.....A LOT!  Poor Trevor....no one has ever let him forget it!
Now on Friday night, we were getting acquainted with Princess Daffodil and Trevor was trying to talk to the new mom very softly while walking slowly and quietly to get a hand on the new princess.  However, Mom did NOT want anyone else touching her new baby....and she made it very well known!! 
Trevor's immediate comment was...."NO MORE!"