From as far back as the Greeks, people have been trying to determine what makes mood the way that it is. What makes us depressed, what makes us happy, what causes hallucinations, the Greeks questioned everything that had to do with the external things that people did. Since then, scientists have discovered more about what makes us feel certain ways and our mental status, but we still have not discovered many things about the human brain. One of the things that we have learned a significant amount about is what makes a person happy or sad. It's simply this: the presence or lack of certain chemicals in the brain determine the mood of a person. These chemicals can be influenced from a number of different things. These chemicals make the difference for a person. Low levels of these chemicals can cause depression, high levels can cause schizophrenia, These chemicals are called Serotonin, Dopamine, and Norepinephrine. These chemicals are just what makes us feel our different moods. Dopamine is the feel good drug that fires off when we do something that makes us feel good. For example, sometimes when we eat food our bodies increase their dopamine levels and we feel good. Norepinephrine has a very large role in motivation and pleasure, but also in movement. Serotonin is responsible for maintaining the balance in our mood. It transmits impulses between nerve cells, functions in the GI tract, and has been proven to be present in bone metabolism, breast milk production, liver regeneration, and cell division. Pertaining to emotions however, it controls anxiety, depression, and overall happiness. Ecstasy and LSD cause large rises in Serotonin levels, which is why people get so happy. Serotonin is a large component of anti-depressant and anti-anxiety medications. Norepinephrine is present in making sure that we can respond appropriately to stress. It's job is to get one into a state of arousal, like in the fight or flight situation to walking into danger, but low levels of Norepinephrine can cause depression. All three of these chemicals are what cause us to feel happy, feel depressed, or feel any range of emotion. With these chemicals, we wouldn't be able to feel happy, we shouldn't be able to feel good, and we wouldn't be able to handle stress. They're important, and without them we would mentally be anxious and depressed. They're a completely essential part of our lives and they help us to be who we are, while impacting our brains, also impacting other parts of our body, and transmitting things to our bodies.