"I reject your reality and substitute my own" is often said when someone is completely unwilling to accept the truth and instead chooses to create their own version of reality. It's often used in a joking or sarcastic way to poke fun at people who are being stubborn or delusional.
Person 1: Dude, you failed the test. You need to study more next time.
Person 2: I reject your reality and substitute my own. I actually aced the test, but the teacher accidentally marked my answers wrong.
Person 1: Uh, yeah right. Keep telling yourself that.
A man working in the capacity of a substitute teacher. Usually claims that he enjoys "teaching" eighth graders how to color when their teacher is gone. Truthfully he is unqualified for most jobs and is able to maintain sobriety long enough to fill a position for the actual teacher who is hung over, having another baby or just needs a break from that grueling 6-7 hour workday for nine months in the year by taking personal days, vacation days or sick days allotted to them while all the way complaining about their salaries and benefits because they've never competed in the real world to know better.
student 1: "That substidude in Mrs. Meyers class is awesome. He sits cross-legged on the desk and we spend the whole period watching movies and talking about his kickass time in college. I friended him on FB."
student 2: "Mrs. Meyers should be coming back in a couple of months, you shouldn't get too attached to our substidude."