why one should not take aspirin empty stomach?

Aspirin, like any other pill, will irritate your stomach. 

When you eat foods, they churn in your stomach and turn into harmless acids that dissolve more foods and digested things, until excreted. So, when you eat something before taking an Aspirin, there are more acids inside your stomach to help dissolve the Aspirin, reducing the irritation that it can have on your stomach and stomach lining.

If you don't eat before taking an Aspirin, you may get some stomach irritation, followed by a stomach ache, but it shouldn't be too severe. But, I would recommend having at least a small snack before taking one, or any other pill, for that matter