Same reason people stayed away from me when I contracted an allergy to a stay over a friends how to their laundry detergent - I had a hell of rash - thought it was poison ivy - you tell someone you have poison ivy - unless the are immune - they will run for the hills - kinda like mono. Best rule of thumb - don't share what you got and in my opinion any kind of disease - or malfunction of the human body - seems to not be socially accepted. How do you think Lepers feel?
Some people always make fun of mono and some look down on those who have mono. This does not happen to everybody, and the best thing is not to allow people to look down on you. Be strong and happy; never mind those who look down on you. People do talk and can think a lot and we cannot control that. Check more information about mono at http://www.webmd.com/a-to-z-guides/infectious-mononucleosis-topic-overview.