He's changed how I used to love my job!

Since the frequent conflict with that new guy in the office, I am more and more demotivated towards this job. I used to love it for the whole year and I was working like crazy with pleasure... the results were amazing and I even got a promotion after 4 months of starting that role... but now... I care less and less everyday... That guy is just killing the fun of this job for me... He's not even my manager, he's a peer... but he wants to control the world and he needs to learn so many things first! He's supper confident, supper aggressive, and extremely rude. Doesn't let anyone to finish off their sentence! He jumps into conclusions without knowing the facts and he uses his personal "assumptions" as "facts"!!