Why do teachers get a kick out of making students miserable?

Okay so from the past classes i have been in (including h.s, and college) i realized a teacher can give a student any grade. Teachers have the choice to have a - and + rubric. As a college student now and a past h.s'er i hate when i see an A on my papers and next to it is a minus . Teachers make us do the most dumb and unpractical assign. And they make us work extremly hard, even the laziest person is cracking open books but in the end they take very little time to grade the papers and give them back with red marks and big fat Ds and Cs. In subjects like math and science i understand because its facts and caclulations but something like english composition how does a teacher sleep at night knowing that he/she gave a student a B+ without taking the required amount of time to grade the paper. They take a day or two to grade 100 students papers each being 5-6 pgs. You might think one day or two is enough but lets speak the truth not all teachers go home that day and work some more at home, they have lives, some go to clubs, parties and whatever weird old teachers who they they're hip do so they cannot have all that time to do it. You know how you hear about a "teachers pet" it is not a myth they are true. I had one in my Art class last semester and she would help the teacher with everything and come early to class and all the works. For me i didnt even do the the persentation at the end of the course, i missed so many days, barely participated in class, i hated the class so much. In the end i got a B+ and i saw that girl around and she got a C-. I mean wtf! Have a heart and atleast let her have her dignity and give the damn C she deserved. Why do they make our lives hell, i understand you shouldnt give a D student C or B, because no one wants a doctor who had a C in college to operate on them but why give a student that has been working his/her butt of and had an A the whole semseter A- DAFQ!!!!