Someone dear to me will often say, “it doesn’t matter” when I express sympathy for something that has gone awry in his day. And then it is a very short journey from “it doesn’t matter” to “nothing matters.”
I believe this is one of the language tells of someone who is depressed. Someone wrestling with depression is much more likely to say something like this than someone who does not wrestle with the dark fog.
There are other phrases as well – and they tend to group under a self-oriented negativity. They use more I centered words and blame themselves for everything.
I say they – though surely I have had my own depressive periods wherein all this was true for me as well.