
What if Harry had:
*tried to feel sorry for someone as clearly unhappy as Snape, instead of deliberately smartmouthing and antagonizing him? How many pre-punishment speeches have featured lines like: "Manners, Potter!" or "Ten points from Gryffindor for your cheek, Potter!" ?
*been willing to hear someone else's views on his father, instead of defending him with arrogance before considering another point of view?
*put hard effort into studying for Potions, and getting at least some of his homework done regularly?
*'fessed up and apologized to Snape about the stolen boomslang skin?
*did not ever Stun him, Stinging Hex him, or accuse him of continued Death-Eating?
*did not laugh in his face when the Marauder's Map called him an "ugly, greasy git"?
*did not spy on him when he was conversing privately with Karkaroff, already stressed out and feeling angsty over his old Dark Mark burning anew?
*controlled his temper, and did not shout, snarl, make sarcastic comments, or otherwise give in to Snape's provocations?
*did not repeatedly challenge Snape's already shaky anger management skills by calling Voldy by name?
I could go on forever. You get the picture.

I have just took out a part that belongs to book five, Try to not put spoilers in your posts
