Mostly, I don’t go for the Melancholy Dane business. Hamlet doesn’t seem to suffer from depression or malaise or melancholy. Sure, he can get a little bit macabre and he does seem to be going through an existential crisis but I don’t think he’s particularly depressive. I don’t think of depression at all in the play.
Except for a line like this.
The depressive, to whom I am closest, says things like this all the time. After a big emotional blow, they will say “It doesn’t matter. Nothing matters.”
Partly it’s that they don’t want to talk about it but it’s also a way to brush away the pain.
Much is made of Hamlet having a sense of foreboding that foreshadows his death and that’s all right here in this scene. He’s got a bad feeling; he waves it off. Horatio suggests he honor it; he waves it off with some of the most poignant lines in the play.
It’s so rare that someone says “it doesn’t matter” when it doesn’t actually matter. It almost always matters a great deal.