Well, I had a "friend" who was not doing well in physics classes, so I helped him out. I helped him out in hw, quiz preparation, and I even gave him my biochem old exams to prepare. The more I help him the more he gets jealous of me. After my junior year, I was suffering from depression, because I realized that I don't have a green card, so I cannot apply to any in state schools, so I couldn't study for MCAT and did tons of research instead. When I told him about this, he was the happiest man in the entire world.
Another "friend" of mine, I helped him out in organic chemistry classes. I advised him how to do well in orgo, let him borrow my orgo kit and textbooks and even showed him my orgo lab reports. As a friend, I helped him out when he was moving too. But he still cannot hide his burning jealousy towards me, and I can tell that he dislikes me so much.
Another "friend" of mine, before he met me, he was not the most stellar premed getting straight Bs in orgo, chem and others. I helped him out in biochem and advised him to do volunteering and research which he never did until his junior year. I don't know why, but this advise and encouragement gave him an intense, competitive feeling against me, and his GPA exceeded mine by 0.05. Now he is talking as if he is better than me in every way. He is still foolish and perverse enough to think that doing 100 hours in volunteering is a lot. I am just too disappointed at him and am not going to help him out anymore.