Culinarily I am somewhat adventurous (eg, I'm willing to try most foods at least once as long as they don't gross me out at first), but some stuff just does gross me out (eg, default amount of mayo on a firehouse sub).

Unopened mayo is shelf stable because of how it's made and sealed, but once opened, bacteria gets introduced. Most of them are killed in a cooking process (if you use it to make grilled cheese sandwiches) but their byproducts are still there and that can still make you sick.