Be kind, for everyone you meet is fighting their battle too.
I just met a customer who started off yelling about his mail and I got defensive. But as he explained (yelled about) the problem he started to cry and told me he was trying to get to the hospital because his 9 year old son had been hit by a car and a cab wasn't coming for another 20 minutes. I went from wanting to smack this guy, to wanting to hug him. (I didn't though).
The title quotation is one of my favorites because it's so true.