Why would anyone be offended by someone wishing them Merry Christmas?

I don't know.  I'm as atheist as they come and I'm not offended by it.  The name of the holiday is "christmas", nonbelievers and people of all religions celebrate it for non-christian reasons, and wishing them to have a merry one is the equivalent of saying, "Have a nice day."  It's stupid to be offended by something like that.  LIKEWISE, it's also incredibly stupid to be offended by the phrases "happy holidays" or "season's greetings" because those greetings also carry the same good intentions of happy times from day to day, and they cover all the holidays from this time of the year, not just christmas.  They're basically saying, "whether you celebrate christmas, hanukkah, kwanza, tet, or whatever else, I hope your days are happy and peaceful."  What's to be butthurt about that?

Not everyone celebrates Christmas. I just tell people "Enjoy your Holidays"

The person could be an atheist.

The same as someone getting offended by you saying f*** Jesus. 

I don't understand either.  Just accept the wishes as heartfelt from the giver.

People not celebrating it such as Jehovah's witnesses 

my dad gets offended by people wishing him “happy holidays”... i don’t get it.