Fridge Horror: As pointed out in First Thunder, the wizard Shazam took a child that life had already figuratively taken a shit on and gave him the responsibility of being a superhero, all the danger involved included, with little to no guidance.
Fridge Brilliance: The reason why Shazam chose Billy to wield that power was because Billy had endured such a hard life, but rather than be crushed by it, he maintained his positive attitude, his hope for future and his compassion for his fellow man. Billy took everything a bad life could throw at him and remained at his core a truly good and virtuous person, which is why even on a planet that included folks like Superman, Billy was the only person Shazam was willing to trust with the powers of the gods. Furthermore, Shazam was on hand to offer Billy any guidance he needed. In a further display of Fridge Brilliance however, Shazam pushing Superman to bond with Billy was just as much for Superman's sake (giving him a true superpowered friend he could relate to) as it was for Billy's (giving him a big brother-ish mentor).