The same things that give onions their unique flavor—sulfur compounds—are responsible for why you cry when chopping them up. According to , slicing an onion breaks open onion cells and releases chemicals that transform the sulfur compounds into a gas. This gas irritates the nerves in your eyes, making them water.
To avoid the waterworks, you can wear goggles while you cook. But if you're not a fan of the whole chemistry-lab look (you do you), simply refrigerate your onions before slicing and dicing. Cold reduces the tendency of the compounds to turn into a gas, which will minimize that burning sensation.