What is it about coffee?

I work for an insurance company.  My job is to assist medical billing offices with claims submitted for payment.  Unfortuanately, you do run into people who are slightly less than pleasant.

I have found that if I drink a large coffee at the beginning of my workday, I am much more capable of taking it all with a smile, and being quite friendly. 

However, I have also found that if I don't have that coffee, I can be a beast to deal with.  Here's hoping my mug runeth over.