kedoshim

The gemara in shabbos tells us the story of a gentile who came to Shamai and said, I want to convert on condition that you teach me the whole Torah while standing on one foot. Shamai threw him out. So he went to Hillel with the same request, who told him דעלך סני לחברך לא תעביד - that which you hate, to your friend don't do -this is the whole torah.
The Maharsha explains over there that he learns this from the posuk of ואהבת לרעך כמוך - love your friend like yourself.  ר' ישראל סלנטר asks, why did Hillel change from the format of the posuk? He should have told the convert דעלך טב לחברך תעביד - that what to you is good,  do to your friend.
Why did he switch it around the loshon to not doing?ר' לוי יצחק מברדיטשוב says on the posuk of ואהבת לרעך כמוך, that it means a person should look into themselves to see all their faults, & realize that despite all one's faults, he still loves himself. So too, he should love his friend despite all their faults. 
The Chafetz Chaim says that we learn from where it says ואהבת לרעך כמוך אני ה that we have to think, how many times did we antagonize 'ה & yet He doesn't take revenge. He still gives us everything good. So too, no matter what people do to us, we have to still do good to them & not take revenge.  ר' ישראל סלנטר says, the reason Hillel switched the loshon is to teach us that doing what is good for you, for someone else, might not be good, if that's not what he wants. 
What Hillel is teaching us is that you have to look to the other person to see what it is that he needs, not what we want him to need. The Torah is teaching us to look to the person's needs, so that what we're doing is really for their good & not just what we think is good but what's really best for them!