One of the real issues we have to realize and account for as humans is that no matter how empathetic, sympathetic, understanding, outstanding, sharing, caring, or loving we are we can only evaluate someone else's circumstances and actions from one perspective; our own.
Now, there are people that are much better at this than others. Ultimately however, the filter of our own biases, prejudices, and experiences limit mere mortals on how much of someone else's issues we can relate to. (see the previous post on Cognitive Bias for a similar thought from a slightly different perspective)
We never know exactly what someone we meet or see is going through or has gone through. Even if you know them intimately, unless you have lived with them and experienced their life as yours (which is almost impossible), you would have no idea. I do not want to come off as "holier than thou", but I try to give everyone the benefit of the doubt. Now, those that know me - especially from earlier in my life - knows that I have not been a poster child for patience. My wife's disease required me to work on and develop some. Things that used to really upset me now just roll off my back. I try to see things from others' perspective. I know that everyone can have a bad day. The cashier that was just short with you may have had a family member die in the past few days and not be able to take off work. The server that is less than efficient may have walked to work because their car broke down and they have no idea what they are going to do. The same can be said for other peoples' evaluation of you and your attitude. They have no idea that you just got horrible news or that a family member is in hospice. Just try to remember this when you are interacting with someone.
My thought when I first thought of this post was this line: One day, somewhere, someone, will remember this time as their "Good Old Days"! The same can be said in reverse. Someone is going to remember this time as Hell on Earth. Your understanding and acceptance along with a smile might make a difference.