I HAVE to be nice in my job, and it carries over into eeryday life as well. I find that, working in the food service industry, as a server, I'm a lot nicer and more patient with other servers at other restaurants when I go out. Unless it's ridiculous, then I tend to get testy.
I try to live by the "kill 'em with kindness" saying too. I run into a lot of crabby people, and I just maintain my smile and calm voice, and usually they chill.
I'm also VERY social, and very much a people person as well. Being that way, I've been able to "read" people a little better over the years.