Let's say we are in a concentration camp, and many of the prisoners are suffering from depression. By administering antidepressants that interfere with the natural pathways in their brains, we are able to make them happier and fully functional, so they can continue the slave labour we have set them. Perhaps the unspoken purpose of such medication is not to make people happier per se, but to make them compliant.
Some common symptoms of depression include: loss of energy, oversleeping or insomnia, loss of appetite, decreased libido, social isolation and withdrawal, lack of motivation and/or enjoyment going about your daily routine, increased uncontrolled substance abuse, etc.
When your body is sick with a cold or flu, it provides an immune response. If you have a fever, it's because your body is attempting to fight the illness. Consequently, you feel like crap, which is actually a pretty good thing at least in the sense that it makes you want to lay in bed all day, which conserves energy and allows your immune system to do its thing as efficiently as possible.
When you have a mental illness (e.g. depression), there is a different type of immune response which may be described by some of the symptoms of depression I mentioned earlier. However, they also have the effect of making you feel like crap, which is actually a good thing because, in an ideal world where we can freely choose when and how we address our problems, we most likely should be taking time to focus on ourselves in order to raise our awareness of the decisions we make which may cause or exacerbate mental illness so that we can likewise raise our awareness of ways we can better care for our mental state. To this extent, Dank is generally correct.
However, we don't live in an ideal world, and we often don't get the chance to take as much time as we want when we want it to address anxiety and depression, or at least not without severe consequences. We usually have so many other responsibilities that we simply don't have the luxury of dropping everything to take care of ourselves. Additionally, when people do get time to themselves, most people do the wrong kinds of things. I would define the 'wrong' kinds of things as passive/escapist activities like watching television, sleeping excessively, using drugs, etc. Instead, active activities such as exercise, meditation or other relaxation techniques or therapeutic exercises, hobbies, reading, learning, etc. are what help us to grow and progress towards self-actualization.
But, if you have a depressed person who already feels run down and worn out, and who does not have the luxury of dropping all of their other responsibilities, the problem is that it isn't very likely that after attending to all of their other responsibilities they're already struggling to manage that they are going to be motivated to consistently do these "active" activities. Instead, people usually resort to the passive activities I mentioned because they constantly feel like they need a break from everything.
So, for those people who don't have the discipline or capacity in their current mental state to both adequately fulfill their typical responsibilities and also adequately take care of themselves, antidepressants can help elevate a person's mood throughout the day so that they feel motivated and energized.
In many cases where antidepressants fail and you see people struggling to ever ween off of them, the problem often isn't the antidepressant itself, but rather that people don't often take advantage of the increased motivation, energy, and optimism and apply it towards engaging in those more beneficial, active activities. Contrarily, those who do are usually the ones you see who are successfully weened off their medications and continue to maintain a positive mental state by abiding by new, better habits instead of the older habits.
Note*: I'm generalizing throughout.
Our answer to problems is to blame the individual. If a fish is out of water don't put it back, but drive a cannula into its veins and pump it oxygenated blood. We treat people so they are functional in their environment, but maybe they should be dysfunctional, and maybe we shouldn't support that environment. Over one in ten Americans is on antidepressants I read. How did we ever cope in the past?