It's probably just to make a joke/light of it, whether you're depressed/suicidal or just joking around. I guess it isn't really the type of thing to joke about but it's a sad fact of life that death eventually comes so you might as well just make a joke of it rather than being sad over it? As someone else said as well, it's become slang in some ways too so I guess people use it more in that way too.