Beats me, people saying "if you're tired enough you can sleep anywhere", making me laugh my exhausted ass off (and feel jealous af, but can't really hold it against them).
I've spent more nights than I can count laid in bed for hours, absolutely shattered, wanting nothing but sleep, and none has come. Then when I do finally fall asleep, it isn't for long or of any decent quality (as in EtraordinaryJoe's case - chronic pain will do that to you), so I'm still always tired, and yet still unable to just fall asleep.
My trick to getting to sleep at night is weed and a meal, to help induce food coma, which again, isn't quality sleep, but it's better than nothing..