Soda has a lot of sugar in it and if you ingest a lot of sugar, you can get a small high from it. But then that results in a low. If you keep ingesting a substance, your body will begin to expect it; but it'll also develop a tolerance to it. So you'd have to ingest more sugar to get that same sugar rush, and if you don't, then your body will make you feel like it's low in sucrose (sugar).
Your body metabolises sucrose into glucose, which you need to have enough of in your blood at all times in order for your body to subsist. Otherwise, your body struggles to do anything at all, including regulating hormones in your brain.
Your body has to prioritise things, and maintaining a healthy mind/mood is far from being the highest thing on the list.
So I guess that's why.