Why do good people get hurt the most?

In psychology my teacher accidentally added 18 points to my test raising it around 10% and I told him because it was the moral, right thing to do. I was from the start when I saw the error, going to tell my teacher that he gave me extra but what irks me is that my friend made a huge deal about it and said "YOU HAVE TO TELL HIM. IT'S 18 WHOLE POINTS!" And I was a bit ticked since she had gotten a 12/12 on an ignment she deserved a 6/12 on and she didn't tell him. But of course, like I always do I told my teacher and he changed it. Then for our midterm, all the questions that were going to be on it were review questions which we had seen on previous test and there is an online site for it where you can practice the exact tests. So I did around 900 questions total since we had a huge break due to snow. And I sent the file with all of them to my friend, mentioned earlier, because I thought it would be nice to help her out since she was still my friend. But we were reviewing during lunch and she was being so ungrateful saying, "ohmygosh, there were so many repeats. like it was so annoying. I had to erase so many of them. and it took time away from my studying. it was so dumb." It made my sort of angry because I had gone to the trouble to send it to her so she could do well. Lo and behold when we took our midterm she got three questions wrong and I got five. And when we were going over it she kept saying "ugh, I knew these two but changed them. blah blah blah. I'm so stupid." And I obviously had a lower score so it made my feel bad about myself. And then we got our grades for the cl that quarter I got a 92.89 and my teacher doesn't round up and she got a 93.00 which is an A. So I want to know what's the point of being a good person, doing the right thing and helping other people if it's going to just hurt you in the end?