To paraphrase Avos D'Rebbi Nassan 16:2:
The evil inclination grows when a person is born, while the good inclination enters at Bar/Bas Mitzvah. As a minor, if he desecrates Shabbos, his inclination doesn't protest. If he murders, his inclination doesn't protest. If he commits arayos, his inclination doesn't protest. At 13, his good inclination is born and begins weaning him off of these sins, telling him, "don't do that - you're liable to death."
Why, indeed, does the good inclination come at 13, and not at birth? Would the decision between good and bad not be more of an even match if he had the inclinations both ways from birth?
The reason I quote ADRN specifically is because of the way it poses the good inclination, not unlike a nagging little brother. It almost makes a person want to sin. Granted, that's the point - to listen to it anyway - but does it have to be so hard? Could HaShem not have made it a fair fight and accomplished the same thing? As it stands, without Hashem's help, the Yetzer Hara would kill us (Kiddushin 30b). Why wouldn't He help us in this manner?