I've said this a few times. Some people think it means that I don't care about anything.
It doesn't matter that we went to a not-so-great place for lunch. It doesn't matter that we spent too much on plants. It doesn't matter that my lovely china mug was broken (by a kitteh, tsk). It doesn't matter.
On the contrary, it means that I care deeply about the things that DO matter. That I have learned the hard way what DOES matter. That by losing my son I now understand that most of the petty annoyances and day-to-day things that bring us down or make us worry really don't matter.
Oh I haven't got this down to a fine art yet but at least I know that when I start worrying/getting annoyed/etc I can tell myself that it DOESN'T MATTER and believe it. Or perhaps just start to try to believe it.
It's a valuable lesson, but one I would sacrifice in a heartbeat.