I might be acting like a rude clerk in my personal relationships too
It is rare to encounter a sullen or unfriendly clerk or representative. If it is a one-time visit or if that is the only place where I can get what I need, I will put up with it, but otherwise, I will likely stay away.
1. Checking the price tag or asking the clerk

When a customer asks various questions without looking at the price tag, they probably want to talk to the clerk. If the clerk is pleasant, things go more smoothly, and one naturally acts that way.
Don't talk to me right now If the aura is like that, the customer will just look at the price tag or the store's pop-up display and be done with it. In that sense, having price tags is helpful, but depending on the pop-up, that very thing could lead to the Don't talk to me right now kind of element. There are also rare clerks who say things like, "It's written on the menu!" or "It's stated right there!" and this is exactly that.
Even if it is written on the menu, the customer may have just overlooked it or not noticed it. Well, I suppose it is debatable whether one should expect such high-level service at a casual shop.
And 99% of these types of clerks have one thing in common: they are unfriendly. Whether they are naturally unfriendly, had something bad happen, or are feeling unwell, the reasons vary.
But one thing is certain: the background of the situation does not matter to the customer. All they see is the result: an unfriendly clerk.
2. Human relationships

As mentioned above, being unfriendly to customers at work is unpleasant for them, and it causes customers to leave.
Now, what about human relationships in general, including work? It is hard to want to interact with someone who is always angry or has a sullen face, even when we have done nothing wrong.
Honestly, isn't it annoying when someone always has a furrowed brow?
Probably, people likely do not gather around such individuals.
Conversely, people naturally gravitate toward those who are always smiling.
However, when we are focused on work or chores, or when we are feeling unwell, we unconsciously make a grumpy face.
This is exactly like the relationship between the unfriendly clerk and the customer mentioned above; for the other person, the background behind the unfriendliness does not matter, nor do they need to know it.
It is clear that when we are busy, lack capacity, or are feeling unwell, we should be conscious of not appearing unfriendly or grumpy to those around us.
3. The habit of smiling daily

To avoid unconsciously having a sullen face and being perceived as grumpy, it is important to make smiling a daily habit. No one is offended by seeing a smile. The only time a smile might make someone even angrier is, if I had to say, when the other person is emotional due to a complaint or something similar.
As the saying goes, "Fortune comes to a smiling gate," so I want to strive to never forget to smile, no matter the situation.

