Too much of anything can be bad for you, Its a saying which reigns true in most cases. There are a number of constructs which we can look at which exemplify this in the real world. When we consider any of the addictions currently recognized it is understood that the people affected are having "too much" of whatever their addiction is. These addictions have adverse effects and negatively impact the persons existence.
A go to could be alcohol or drugs, both of which when overindulged in can result in the body shutting down and death. I know, some people will be saying "well duh!" they are the "bad things" of course they will kill you, but what about water? After all we are made up mostly of water and we are often told we don't drink enough of it. However, drinking too much water has adverse effects on the body, over hydration leads to hyponatremia.
This is not to say that drinking alcohol, gambling, drugs, drinking water, oxygen, etc. is bad in itself.... quite the contrary in fact, many of these have positive effects on life if we interact with them in moderation, and some are absolutely necessary for our existence. What we may not be acknowledging when we say that "too much of a good thing is not good for us", is that excessive use or abuse of something can be detrimental to the self.
The bottom line is that balance is the key, this is exactly how the human body works as well. A constant quest for perfect equilibrium... Homeostasis. Unfortunately, there is always work to be done and we never really manage to reach a point of perfect homeostasis within the large range of variables which make the human existence.
If we look at a single variable, such as the wake and sleep cycle, we can see that when we wake we move form a state of slumber and get more alert as time moves on, as we use our energy and go about our day, we move towards being tired and sleepy. While asleep we recover form fatigues of the day and begin the cycle again. However, other variables (for instance hunger) maybe at another level when tiredness reaches the central point of balance. As close as it may seem that we are in a state of Perfectenschlag, we are often be far from it.
Being tired or hungry can feel bad, it makes us want to remedy the situation with a nap or a sandwich, that is the purpose of those sensations. At the other end of the scale being full (of food) or alert serves different purposes, to stop eating and to use energy. The understanding being that no matter what we are experiencing, regardless of what we feel about the sensation itself, serves a purpose.
This is also true of Anxiety and Depression, often we see these states of being as something which we do not want to experience, that these sensations should be avoided, if you have them then there is "something wrong with you", but this is not the case. These experiences actually serve particular purposes in our growth and evolution as a species.
Anxiety can be generally defined as feeling anxious, nervous, worried, etc. about the future or future events. Now this sensation can be uncomfortable, at extremes it can cause panic attacks and varied forms of somatization. However, this is the result of our overindulgence in the sensation. On its own a degree anxiousness does serve a purpose, lets use a job interview as an example.
Anxiety
Many people have anxiety about job interviews, it is after all one or more people constructing a concept of "your worth" and then making a decision on "if they want you" based on that concept... even if one does not feel anxiety about the situation themselves, one can sympathize with those who get nervous about this experience.
But lets look at what this means, for starters it means we care about it, we care enough to think about it long before we have to "do the task". Thinking about it is "visualization" of the situation, similarly it is like running simulations in the mind of how the event will go, what we should do and say. This is us preparing ourselves for the future to the best of our ability. Used appropriately anxiety is a tool to prepare us for future events and gravitating towards a particular outcome.
Depression
Depression can be generally defined as sadness, low mood, unhappiness, etc. for events which have transpired in the past in our memory, but the effect persists in the current and future states. Again this is an unpleasant feeling which can have incredibly negative impacts when experienced for considerable lengths of time.
But lets look at its purpose within our lives, feelings of depression indicate that there is something which has adversely affected our state of being, that something is wrong (or went wrong) in how we want to interact with things in our lives. This gives us the ability to learn from our mistakes, make adjustments to make sure these things do not occur again in the future.
Depressed about going through "another" crappy relationship after it has ended? looking back on it can help prevent the next one having the same outcome. Feeling sad about being bullied at work, dissecting the situation of the wrongs done can be the catalyst for building the case against the bully.
Something i have used recently is the "Pebble in the shoe Analogy", that is, these sensations we consider to be adverse is much like pain in our lives. Pain too serves a purpose, its there to let us know that something is wrong, that action must be taken to prevent further damage. It is not the pain which we must focus on but the cause itself... the pain is merely a byproduct which is eliminated when the core issue or the source is dealt with.
The core issue/source is like a pebble in ones shoe, it creates a sense of pain and discomfort to the body, these signals hit the brain and we know that we must remove the pebble from our shoe to stop the current pain. If we deny or ignore the pebble and pain we are likely to cause more damage to ones self, such as bleeding, blisters etc.
Similarly in areas of excess anxiety and depression we find that the focus tends to be on the pain and discomfort being felt (the pain) and not the root cause of the sensation (the pebble). However, if we can shift that focus to the pebble we might just be able to make some changes which can shift the paradigm completely. This is not to say that there are not cases which are outliers, falling beyond the scope of this particular construct.
Conclusion
While Anxiety & Depression may be considered as particular negative experiences, the purpose they serve is as guidance in our lives to gravitate to a greater sense of self. Used appropriately these sensations help us define ourselves, what we want, how we want it, etc. All of us have experience with these sensations on a daily basis at varying levels, it is indeed part of being human.
However, when we focus too intensely on these sensations we can get stuck inside them, we have "too much of them" creating an abusive relationship with the self. Excessive, repeated and extended exposure to the Anxiety & Depression beyond their intended purpose can put us in a self deprecating loop of psychological distress.
It is perfectly normal to experience Anxiety & Depression, but it is how we interact with Anxiety & Depression that makes all the difference. It is up to us how we choose to interact with these constructs, some of us may naturally be better equipped to handle these situations, some of us may have more on our plate at a given moment, this is the argument of nature and nurture. But none of us can deny that we all need help sometimes and if anyone reading this feels overwhelmed by Anxiety & Depression i strongly recommend reaching out to someone who you trust a medical professional or the varied local programs, hotlines, etc. designed to assist those in need.
Australia
https://au.reachout.com/articles/support-services-for-depression
America
http://www.mentalhealthamerica.net/contact-us