Broadly speaking, with the exception of those who are clinically insane, things are said or done for a variety of reasons rather than "... for no reason in particular".
"Rudeness" is not really an objective characteristic of personality. It is a derogatory, and very subjective, judgement made by someone who takes it upon themselves to criticize another person's interaction with them (or others), often with the intention of imposing a measure of restrictive control, or in an attempt to acquire a degree of social superiority, over them.
Example 1: The parent blethers on incessantly about something, and if the child attempts to say anything whatsoever (relevant or otherwise). The parent raises his voice saying: "Don't you interrupt me when I am speaking! I can't abide that sort of rudeness."
The parent is clearly trying to exercise a control, over the child, oblivious to the fact that: if "interrupting" is indeed rude, he is himself being just as rude by interrupting the child, and by his own example, further training the child to do likewise in future.
Example 2: Individual A says: "That's bullsh!t" Individual B retorts "Don't be so rude! It pisses me of when you use foul language for no reason in particular."
..... garnering for herself a grandiose exemption from the social niceties that censure "rudeness", by using the taboo phrase: "It pisses me off", herself.
We can all be rude when we feel like it can't we?
At least, we can when we have a reason to do so.
yes. yes it does.