Dear Happy Ass Co-irker:
I am not a "good morning" person. Yet everyday you say "good morning" and then look hurt when I don't respond or when I grumble at you rather than picking up on the very obvious cue telling you that I am not a "good morning" person.
I shouldn't have to BECOME a "good morning" person to spare your feelings. What would be better is if you accept me for who I am and stop saying "good morning" to me in the hopes that one day I will respond. Stop doing the same thing expecting different results.
In closing, you leave me alone and then we're both happy.