I play a game in the library where I work. It's called 'Smile, you miserable bugger!'
I just pick the rudest, most scouling, miserable git I can find, and if they don't return my cheerful and welcoming smile (which everyone sees once, but only happy people see again) then I grin and grimace and utter the pleasantest of pleasantries in the most phoney manner possible. If they STILL don't smile back, then they are elevated to official Miserable Bugger status, I cease to smile at them and I conduct all future transactions in total silence.
Don't mess with a shunned Tigger!