It may not be useful, or a life worth living, if you go about your usual day imagining the worst is going to happen. That sounds like a bullshit kind of life. But imagining tragedy–the things you most don’t want to– can lead to you being prepared for (and happy with) anything that happens in your life:
It may be useful, however, to at least imagine the possibility of your dreams never coming true, your goals never being realised, the people (or animals) you love disappearing, and everything else you hope for never happening. Imagine that trip you want to take, that place you want to live, that dream job you want to one day get, that family you dream of having… imagine none of it happens. Who are the people closest to you? Imagine they all just weren’t there tomorrow. Your job, your house, your hair, your most valued possessions… imagine losing it all. What are you most thankful for in your current situation, imagine: ALL GONE.
And why the hell should you do that?
A) because it is possible (and some of them are inevitable) those things will happen, B) it can show you that even the toughest thing are bearable, and you can adapt to anything C) the things we most hide or divert our attention away from, are often the things we most need to address, and D) no matter how ‘bad’ your situation is, there is always something to be grateful for. Unless you’re dead, but in that case you have nothing to worry about anyway.