Most of the time the things you convince yourself to laugh at are not laughable at all. They are painful reminders of a past that was ruthless in its ability to devastate you till you were reduced to less than nothing, till you curled up into yourself, both in the physical and the psychological parts of your existence. Till you cried until there were no more tears. You laugh because it is easier to recall the ridiculous in all of it, as opposed to the painful.
You laugh because you have yourself convinced that nothing will ever get through to you that way again.
Except that it's not funny.
It's not funny to remember how it actually was.