Why do i have periodontal disease at 19 years? didnt like to brush so got plaque but no cavities.

You didn't brush: Periodontal disease (disease of the teeth at/below the gums) is very serious and common in those that do not floss and brush. Once you get it under control you may want to consider brushing with a soft brush twice a day and flossing once ever day.

Bacteria: Both cavities (caries) and periodontal disease have bacterial origins: the processes involved are different however, and though you may have been resistant to caries, bacterial plaque (and different strains of bacteria) may & have caused your periodontal problem. So, bacterial strain, anatomy of the tooth, tooth contacts, genetics and perhaps some luck may be at play. It is preventable! good luck!

Aggressive disease: Periodontal disease that occurs in someone your age is defined as aggressive in nature. You have particularly virulent (think nasty) bacteria that have the potential to produce a lot of damage quickly. Or - you may have excess gum tissue complicating your hygiene efforts. A periodontal specialist is the one to see for diseases of an aggressive nature.

To Brush or Not: You do not have to brush all your teeth. You only need to brush the ones that you want to keep. The bacteria can cause cavities and can cause gum disease for which you can loose teeth from either or both problems.