In terms of dentistry, bleeding gums only really occur because of one main reason: gum disease.
The number of adults in the U.S. that have some form of gum disease is pretty staggering. Between the ages of 30 and 65 over 64 million adults have some form of gum disease. The percentage of those with periodontitis increases to about 70% with adults over the age of 65.
Gum disease itself is basically an infection of the gums. Just like other parts of the body, when infection occurs, the gums around the teeth become tender, swollen and red. The infection is the bacteria forming plaque and dental calculus on and around your teeth.
The body, in general, does not like infection. In the case of the gum disease, your body doesn't like it so much that it attempts to eject the "source" of the infection (your teeth) right out of the mouth. The bone surrounding your teeth will begin to resorb (dissolve) in an attempt to get rid of the teeth with the heavy load of bacteria on them. The jaw bone resorption will in turn lower the level of gum surrounding your teeth and actually creates "pockets" around the teeth that are infected. Bacteria will go into the pocket and stay there. You can't floss down there, you can't brush down there. Those bacteria just stay in the pockets and keep the infection alive.