Why do two things I like to eat sometimes taste so bad when eaten together?
—R. Lange, Houston

Biosciences professor Tim Jacob, who studies smell and taste at Cardiff University in Wales, mixes up an answer:

Among the five tastes, salty, sweet and umami (meaty or savory) are appetitive, driving us toward essential nutrients, whereas bitter and sour are aversive, alerting us to potentially harmful substances. Mixing the aversive with the appetitive sends conflicting information to the brain, and confusion is what the senses are trying to avoid as they supply you with useful, lifesaving information. This mixed signal is why you reject food that has gone off. You do not want to eat a blend of the good and the bad.

Yet consider the phrase “sugaring the pill”: pills are medicine and as such are poisonous in large quantities. They thus taste bitter but can be made more palatable by a camouflaging sugar coating. Similarly, coffee can be improved for people who are sensitive to bitterness by masking its sharpness with cream or sugar.

As adults, we can override these warnings and acquire tastes for coffee, olives or strong cheese. But you will confound your senses if you mix a formerly aversive taste with an appetitive one. (Care for some pickles and cocoa?) There can be delight in the confusion, however: sweet and sour is a popular choice in Chinese cuisine.