As humans, sometimes we're not as sensitive to each other as we need to be. We tend to lash out to strangers when we're having a bad day, we give people the middle finger when we're stuck in traffic, and sometimes we get aggressive when things don't go perfectly our way. And we justify these actions at times with excuses like "Well, I was having a bad day..." But that person you were hurtful towards will now remember you in a negative way—and while they might not cross your path again, it's still important to at least be kind, no matter the circumstances.
We come across new people every day—whether at work, in class, on the street, there's always an opportunity to either positively affect someone or make their life more difficult. We can't read minds; there's no possible way of knowing what is going on in a stranger's life. But as human beings, we all have similar experiences. That old lady sitting by herself at Starbucks has lost someone valuable to her. That cashier at Target has had her heart broken, probably more than once. The waitress at your favorite restaurant still has to pay her rent by the end of the month. That guy that sits next to you in class might have to work two jobs when he gets out of school. My point here is that we truly have no idea what kind of struggles the people around us are experiencing. Yet for some reason, we overlook the possibility of others' suffering because we are blinded by our own problems.
We don't consider the fact that other people are going through their own everyday battles because ignorance is bliss. It's easy to treat someone poorly for messing up your order—but being patient and understanding will take you so much farther. Don't assume that everyone has a perfect life. Assume the opposite. Just because you don't know what's going on doesn't mean that it's nothing. And just because your problems seem huge to you doesn't mean that they make everyone else's smaller. Make it a goal to be more aware of those around you; try your hardest to hold your tongue when you feel like making that spiteful comment. It might be nothing to you, but you leave that person with an impression of you that they will never forget.
"I don't care what other people think about me." We hear this all the time, but this is part of the reason why so many people act rudely on impulse. This is why so many people think that it's OK to lash out at others for petty things that could be resolved without confrontation. It doesn't mean you should compromise who you are to impress others; it doesn't mean changing who you are to be accepted. It just means taking a moment to ensure that whatever you say reflects the inherent kindness in your character. Be aware that whatever problem you're going through isn't unique to your existence; there are billions of other people on this earth right now with problems of their own, not including those who came before you and those who are still to come. So gain some perspective, get out there and make someone smile today!