It's an effect of human evolution. Recall that until very, very, very recently in human evolution, we had NO idea what was in the food, and only had sight, smell, touch and taste to go on, with taste being the major contributor and last line of defense of knowing what we ate was good for us or not.
While we have 5 basic flavors our tongues can pick up, many of them are there as warnings to warn us what we are eating might be bad. Bitter is a great example of this, for the most part people avoided bitter foods because they were associated with natural food that were not necessarily great for us. Over time, as we found foods safe to eat with bitter flavors, a taste for them developed. Sour could also indicate if a fruit for example may not have been ripe yet, yada, yada, yada..
Sweet is different. Sweet does not necessarily indicate the food is safe, but it does indicate the presence of sugars in the food which are required from the body in order for it to survive. Thus the desire to find food that is sweet is very strong, because in essence that is what our bodies are looking for when it is hungry. There is really no "nutritional" benefit from the other flavors, only sweet becuase it targets the flavor of sugars which our bodies must have.
So, while we can detect other flavors with our tongue, the brain is most "pleased" with sweet because that is how our brain was rigged to be from an evolutionary standpoint.