Thanks to the receptors in our taste buds, eating is one of life's greatest pleasures. Of the five different taste sensations, sweetness is probably the most pleasing. Starting from infancy, our bodies are already programmed to seek out sugar because it provides the energy to keep us going. There is also some indication that babies can be influenced to like salt if they are exposed to salty foods early on. As part of an evolutionary survival mechanism, our bodies are programmed to avoid eating bitter tasting things because many toxic compounds taste bitter. People like to eat sour-tasting foods, like citrus fruits or pickles, but from an evolutionary perspective, sourness is generally a warning for food spoilage. Finally, umami is supposed to be a "savory" taste, which is produced when our taste buds detect glutamate, the salt of the amino acid glutamic acid. Foods that naturally have an umami taste include cheese and tomatoes, so it's no wonder that pizza tastes so good! Here are a few more tasty tidbits.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post