The tennis shirt / polo shirt is the most casual shirt I wear. I don't wear them too often, I only wear them certain appropriate times, which in my opinion is when I am doing some sort of sport / activity or working on something around the house. There is of course, always an exception, and so there is still the very casual occasion I will wear a tennis shirt / polo shirt outside of sport or work. When I talk about work, I mean repairing something, yard work or doing some sort of potentially dirty job. You don't need to worry about getting your tennis shirt / polo shirt dirty, it is 100% cotton and practically any stain will come out. It isn't a formal shirt. It is meant to be casual. The pique tennis shirt / polo shirt is like the OCBD it looks better and better (and feels better and better) the older it gets.
It is hot and humid here. I was working on installing light fixtures in the morning and so after lunch I cleaned up and changed into a fresh clean shirt. I have a full arsenal of tennis shirts/ polo shirts. Because I wear them for sport or work I think of them almost like many people think of tee shirts. I have a bunch of them always at the ready. I didn't buy all of them new. I found many at thrifts and on eBay. In fact, it's often better to find them used. As is the story in many other cases, the material they were made of years ago is thicker, stronger and better than that which they are generally made with today. I will almost always prefer a vintage tennis shirt / polo shirt over a new one.
I think a lot of guys have a hard time making the leap to truly thinking about their tennis shirts / polo shirts as a tee shirt. They still think of it is as more, as something nicer. It isn't their most casual, most relaxed, they still have a tee shirt there waiting. I think that if one can get to the place where the tennis shirt / polo shirt is one's most casual shirt, one's wardrobe levels up a notch in terms of formality and total intentionality. Essentially your basement is no longer the basement but it is your first floor that is the basement. The basement has been filled in with soil. If tee shirts are off the table then everything has shifted upwards in a good way. This doesn't mean you are less relaxed, it means that you become more relaxed in less casual / more "formal" clothing. The prior relaxedness one felt in tee shirts because the relaxedness one feels in tennis shirts / polo shirts.
Eventually you will be living in your tennis shirt / polo shirt as you once lived in your tee shirt. There is something just much -for lack of a better world - cooler about doing work around the house or some dirty job in a shirt other than a tee shirt. There is something almost too careful about yourself if you have to resort to the bottom of the barrel when you are doing something that may get you dirty. It's almost like you wear the classics only up to a point, and then you go to the tee shirt. There is something more very not-caring (even if you do actually, the perception and feeling is different) in a great way about still wearing a shirt with a little more intentionality, even when doing something on the potentially messier side. It is about looking great and intentional every moment of the day, not just the "clean" moments. Of course, it could be interpreted the other way around as actually being more uptight, but it isn't. It is shifting the natural life upward, clicking it up a notch. It is removing the option of the tee shirt and expanding a natural dignified sense of living in one's clothes.