Why shouldn't you take aspirin if you have an ulcer

Because ASA causes: stomach bleeding and can make an ulcer much worse

Two reasons: Aspirin has a direct potentially erosive effect on the stomach lining causing the lining to become inflamed (gastritis), especially on an empty stomach and thus can contribute to creation or perpetuation of a breach in the stomach lining (an ulcer). It also inhibits platelets which are important for clot formation which is needed to stop an ulcer from bleeding. If you truly have an ulcer see a doc