system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Serotonin is the hormone what makes us happy, so why aren't we just injecting it into our body/taking it as tablets to get out of misery on command?
gpt
Hormones like serotonin are like ingredients in a recipe. You need them to be in the right balance for a dish to work.

Imagine serotonin as sugar, if used sparingly, you would enjoy it as a glaze or sauce in your entrees and finish your meal with a sweeter dessert. If you dumped a container of sugar on every dish of your meal, none of it would be enjoyable. 

People's bodies, like their tastes, differ as well. Some people like their dishes sweeter while others do not. Therefore a set sweetness for one person would not be enjoyable for others. This is why it is difficult to find the right medicine to treat neurological diseases like depression because every person is different.

The body also has limited tools to communicate with other organs. Serotonin affects mood in the brain but also appetite and digestion in your digestive system. Some of the side effects from drugs increasing or decreasing one hormone is due to the body reusing the hormone for many functions.

Lastly, the body is naturally ~~lazy~~ efficient. If it's getting a hormone from an injection or pill, it is less likely to make it. Therefore, when a person stops taking the injections or pills the body goes into withdrawal because it stopped making that hormone, causing other symptoms. This can cause a dependence on the substance, otherwise known as an addiction.