Lindy,
Lisa is right that people can do fine with platelets of 100. That's about 2/3 of the low end of the normal range for both men and women. If they don't get much lower, and your other counts stay normal, putting up with a tendency to bruise easily isn't going to stop you from living your life the way you want. Perhaps the doctor will advise you to avoid wild roller-coasters and give up your plans for a pro football career.
Medically, low platelets can result from a number of causes. It may be that your bone marrow is producing an insufficient quantity of mature platelets, but it could also be the case that you produce them but they don't all survive in your bloodstream, meaning that some of them are being destroyed before their normal lifespan.
I'm glad that your doctor is taking the conservative approach and not putting you immediately through a battery of tests other than routine blood count checks, although in the long run further tests may make sense, especially if anything changes. Having somewhat decreased platelets without other symptoms isn't cause to panic but it's a good reason to keep monitoring your counts and take note of any other health changes.