Why do I have bleeding gums?

Here is the short answer:

Every time you eat food is forced between your teeth. Sometimes the food between your teeth is big enough to feel but most times it is not. This food will either be removed by flossing and brushing or it will stay between your teeth, in a nice warm moist environment – until it breaks down into bacteria and acids. The acids cause tooth decay and the bacteria causes gum disease.

The first stage of gum disease (also known as periodontal disease) is gingivitis. Gingivitis is an inflammation of the gum tissue caused by bacteria wherein the gingival tissue becomes red, swollen and forms blood blisters inside the pockets of gum tissue surrounding your teeth (periodontal pockets). When these blood blisters are disturbed by either brushing or flossing they break open and blood rises to the top of the gingiva where you can see it.