Actually, once you've had chicken pox you're not done with the virus for the rest of your life...it's hunkered down in your nerve roots waiting for an excuse to pop up. And while we're on the subject, obviously your immune system HASN"T destroyed it, has it? No, it hasn't...and if you have an outbreak of shingles you are contagious, and you can give other people who haven't had it chickenpox. Approximately 2 people die per week in the US from chickenpox. (CDC) Not a lot of people, but you'd hate to be one.
(and mono may not have been a big deal for you. but I guess you're not one of the people who ended up with a ruptured spleen, Bells palsy, hemolytic anaemia,myocarditis, aseptic meningitis,encephalitis or Guillain-Barre syndrome from it.)