Our bodies are in a constant change. Sometimes those changes might tip our physiological scale quite unexpectedly. We assume that we operate on some kind equilibrium and thus if the assumed balance is disturbed abruptly that would indicate to most us a problem that owes to be solved. Such is the case with most of our assumptions about psychological disorders. We believe that "chemical juice x" is off balance that causes us to react in a certain away. This pattern of thought though is false.
The simple reason why a chemical imbalance is impossible is simply because there is no basis from which we can draw an "average" sample. For example, we know when the PH of a fish tank is off that might lead to fish death. Thus, we lower or increase the PH so we can meet the ecosystem's average requirements. For human beings, no such thing as "average behaviour"exists. There is no average human from whom we can draw comparisons. There is no "average physiology" to emulate in order to reach a "chemical" or "neurotransmitting" balance.
Unlike traditional medicine where specific values can indicate different things, in psychology there is no direct causative factor for determining behaviour or mood. For example, we can assume risk factors for heart conditions if someone has high blood pressure, high cholesterol and they are overweight but even in this case our estimate can be rough and not scientifically demonstrable. Exceptions to the rule such as fat, unhealthy people outliving thin healthy people with no risk factors render our scientific hypothesis false. In science, something has to be true every single time it is tested. Not sometimes. This is how we get the laws of nature, such as gravity. This why our perception about diet and eating habits and how those might affect our health have changed greatly over the past few years. The evidence are very inconclusive to the point of becoming irrelevant when it comes to following a preventive course of action.
In traditional medicine to deal with such error discrepancies we rely on different risk zones. Our physiology is dependent on some parametres and this if we assume control over those our health can be improved. When it comes to our behaviour though the situation is much trickier. There is a vast array of behaviours and influences one can experience and in combination with out physiology it can produce unexpected results. No single neurotransmitter or hormone can cause a definitive change in behaviour. No medication can "balance" anything in our body in case our lab exams seem off. We can administer oxytocin to someone but that can cause other changes in the body and further "imbalances". Our body physiology does not work with on/off switches. We cannot reduce one "chemical" and increase another in order to "balance" things out.
Those who try to attain a "balance" in behaviour rather work out a social engineering program. If we get enough people to behave in a certain way and accept such behaviour as "normal" then all the deviants should work towards that direction in order to fit the norm. Such is the case with psychology and general psychiatry today. We model behaviour based on a client basis. If the individual feels that they have a phobia towards something or a "disorder" and they can't handle it, then we administer what seems to be an appropriate medication in order to "solve" their problem. Thing is, this is not science. This is merely trying to satisfy someone that has a very unique problem with a very generic approach.
Depression and Anxiety are perhaps the most central problems in the dialogue about "chemical imbalance". Thing is, both are entirely natural occurrences. The body experiences depression and anxiety much like it experiences happiness. They have gotten a negative connotation due to the cultural narrative reasons, not medical. We get depressive and anxious due to our lifestyles much like we get happy. Sometimes our bodies signal that we need to change something and thus we go under the anxious or depressive mode. No medication can balance our day to day behaviour. We live in a society where we view every single unpleasant thing that happens to our bodies as a problem that needs to be fixed. If we approach it as a "ride of passage" then it becomes an entirely different thing.