What I should say – “But what if it has happened, happened with extreme prejudice. The worst thing in the world happening in the worst possible way at the worst possible time. Would I still have to cheer up then? Would you still accost me, a total stranger, in the street and insist that my misery shouldn’t be a burden to you? Insist that I put on a fake smile, as if that’d cure all my ills, ills about which you’re totally ignorant?”
What I actually do - give a weak grin and walk on.