I can just imagine Shazam's thought process here. "I need someone to give nigh-rivalless power, fueled by the very gods themselves. Clearly he my champion needs to be mature enough not to abuse this power and compassionate enough to use it for good.

...I know! That bitter kid who lashes out at people who are trying to welcome him and give him a home, a life, and companionship. Wisdom of Solomon, you've done it again!"