I guess it's one of my pet peeves of people who dress really slovenly (in the old days people used to dress up for travelling on the plane) these days.

It's OK to dress casual as long as it's neat, clean and you are groomed properly. I avoid shorts and sandals because of the A/C. I try to wear deeply patterned shirts or at least avoid white shirts knowing that I tend to spill things on my shirt whilst inflight.

I also agree that if you are dressed up a bit, and act in a professional manner, staff tend to treat you better, which may come in handy when you need their help (e.g. when flights are delayed, canceled, etc.).