What is a Sweet Tooth, Really?
The simplest definition of a sweet tooth is that it's a craving or desire for something sugary or sweet. Although the reasons behind the how and why of a sweet tooth are based in science, in many ways they still aren't totally understood. In studies rats were shown to develop addictive behaviors when fed a diet rich in sugar, and they experienced withdrawal symptoms when the sugar was abruptly taken away. Based on this scientists discovered that sugar stimulates the brain to release dopamine, a 'feel good' brain chemical that gives us a natural high. It seems that it's that natural high that makes sugar so appealing. Theories are that the brain gets accustomed to the raised dopamine levels and that natural high is what the body is really after when a 'sweet tooth' starts acting up.
That's not to say that sometimes you don't just want something for the pure flavor and enjoyment of it, because of course that happens too. And I think some of our cravings are based in our lifestyles and upbringing -- sometimes you crave something for the 'warm fuzzies' and comfort it brings based on childhood memories and old habits. And of course sugar isn't all bad -- having it in moderation is not only not harmful but some sugar is necessary to life.