Everybody has been impolite at some point in their lives. Contrary to what some would like to think, we do not all pop out of the womb pre-programmed to understand every single social rule and expectation that will get placed on us throughout our existence.
Still! So long as we have the mental capacity, it's important to show a basic level of civility to our fellow human beings. What this could mean can differ depending on upbringing and personality, but it can sometimes feel like there are individuals out there who just don't think about this kind of thing at all.
This has been the subject of an infuriating Reddit thread, in which Redditors describe all the things that are bad manners but people don't seem to realize it. To be honest, many of these examples are also the perfect illustration of those who should know better but choose not to.