Sometimes we interact people and it seems relatively inconsequential to us. Maybe we are checking in for a flight and we briefly speak with an agent. Maybe we are leaving a store at the same time as someone else. The possibilities are endless. Perhaps we will never see this person again, but perhaps we will. The bottom line is that we never know. Every single interaction we have does leave an impression on both people, some bigger than others, so think about this when interacting. Perhaps that store clerk was rude to you, but you can be nice back anyway. What is the harm?
You always make an impression. It cannot hurt to make it a good one.