"Thou tellest my wanderings: put thou my tears into thy bottle: are they not in thy book?"
Treating everyone respectfully, kindly, and considerately is a good general rule. There are many things we will never know about the people we deal with: the cashier at the grocery store, the mailman, the waitress at a restaurant.
Most, if not all, people have a secret sorrow, physical or emotional: a sore foot, a cough, a back-ache, an injustice, a broken tooth, a heartache, insomnia, and the list goes on. They may smile and try to be brave, living their lives as joyfully as they can in spite of the problem, but the pain is still there.
Even the apostle Paul had a "thorn in his side" which he prayed the Lord would deliver him from. God said, "My grace is sufficient for thee, for my strength is made perfect in weakness."
Be kind to your fellow humans. Try to encourage and lift them up. Everyone has a pain or secret sorrow.
There are three types of people in the world: those who tear down, those who do nothing, and those who build up. Help others along life's way; encourage others. Be a blessing in this life.
"Only one life; twill soon be past. Only what's done for Christ will last."