Instead of just living a happy life and enjoying themselves, Atheist are argumentative, prideful, insulting, and overall miserable people. Why is that?
Methinks the questioner may have had a limited experience with “Atheist” (just the one?). Many atheists are truculent; theists can be too. Many atheists are amiable pussycats; theists can be too. Some atheists bristle over the violence and scientific stupidity that has shown up now and then among religious cultures, and may get as testy on it as protesters at a Planned Parenthood can get. Or, if you want more nasty, those Westboro Baptist jerks making a nuisance of themselves at military funerals. And how much pridefulness was there at the average medieval auto de fe as the religious did in their heretics? I guess a smidge, maybe.
As for the argumentative part, the rejection of a particular faith cannot help be argumentative if dealing with someone retaining that faith. Comes with the territory. Some theists (not all, by any means) take any criticism of their beliefs automatically as an insult, because it’s stepping on their “sacred.” That too comes with the territory.
Miserableness is an interesting variable to measure. If our questioner has a study on this documenting the incidence of misery among those of religious faith, and those not, it might be nice to share. Work I’m aware of appears not to be so cut & dried: https://link.springer.com/article/10.1007/s10902-007-9045-6.
If your “atheist” is genuinely a jerk, by all means have them strike up a chat here, so that a bit of civility (and maybe wit?) might rub off on them. Maybe.