Many causes. Brusing may be normal, and inherited genetically. It may also be an indication of a vitamin deficiency, or a deficiency in the compounds needed for blood clotting. Your doctor can run simple blood tests to determine the causes of bruising.
Varies. Can be related to age, steroids or chemo. Can be due to inherited things like von willebrand's disease or hemophilia. Aspirin and other blood thinners, including supplements like garlic. Liver disease. Low platelets from spleen prob. Or could just be normal variation. Check with your pcp. May examine your bruises and order some lab work. Hematology referral if necessary.