There are the tough customers and tough days!
2 things I encourage and have done:
One of the big benefits of our career is that you can go on "walkabout!" When a resident has just sandblasted you with negatives you can go walk vacants, walk your property, go to the bank, shop for office supplies, go some happier place and BREATHE!
The 2nd is that, YES, there are those grinch customers (as Anne Sadovsky has said, the ones that put the CUSS in customer service!) But there are the ones that have blessed you as well! I like to write down the stories of the ones that brought cookies, left a note thanking the staff, shared a life moment and you were able to help them...Keep a journal of those moments and read them on the days you are afraid you may go postal on someone!