We can make this argument by pointing out at least two very important points.
First, we can say that treatment for depression is not something that is really elective. It is not like a nose job or any other procedure that can typically be neglected without serious harm to the person. Therefore, these treatments are very important.
Second, we can point out that it would be hard for some insurance plans to carry this while others do not. Those that did not cover these services would have lower costs and be more popular among most people. That would leave only the people with psychiatric problems buying the plans that cover those conditions. This would make the costs of those plans very high because the cost of treatment would not be spread over a large pool of people.
Using these two points, we can argue that insurance plans should be required to cover this treatment.