Why do some drivers egos get to them when trying to pass them?

This is last night after I finished a job at a night delivering mud to a rig that requested it, while going to the disposal to get my trailer washed out i attempted to pass this other driver doing around 65-70 on the interstate, so I decided to pass them well I'm guessing me attempting to pass them in the left lane got there ego bruised when I started to pass them doing 75mph because they decided to speed up and sit next to my trailer doing like 73 or so and not let me pass them while traffic started to build up that's when there ego let go and let me pass them, all I wanted was to be in the lane for my exit but they're ego was I must stay in right lane no one must pass me, honestly ridiculous behavior