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.
Freddy's bad leg probably has him in near-constant pain, and his gait as seen in the movie trailer makes you wonder what sort of access to physical therapy he has as an orphan in various circumstances; for the New 52/movie version, add in a likely solid fear of painkillers since there's no way a family of five foster kids wouldn't be affected by the opioid crisis already.
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).
In the live-action series, Mentor claims to be old enough to have taught archery to Cupid, and has said that his name is actually Mentor. Could he be the same Mentor from Greek mythology?
The entities who give Capt Marvel his power Solomon (wisdom), Hercules (strength), Achilles (courage), Zeus (power), Atlas (stamina), Mercury (speed) two are actual gods (Zeus, Mercury), two are exceptionally gifted mortals (Solomon, Achilles), one is between god and mortal (Hercules) and one is a Titan (Atlas). In the Trials of Shazam, Atlas is killed and Apollo the god of healing takes his place, though that doesnt appear again. This represents that Capt Marvel embodies and wields the powers each mythological figure in SHAZAM carries.
During the Trials of Shazam, Freddie Freeman visits Solomon and Achilles first, both notably human figures with amplified wisdom/courage. They test Freddies humanity to see whether he can handle the strain of being respectively smarter and braver than the average person. The other trials come down to physical abilities, which any superhuman being possess in some measure.Thus, whoever becomes Capt Marvel has to be smart and brave enough to wield the power of gods, but before that he must also be a good enough human being that this power won't corrupt them.
Freddies rival Sabina never visits Solomon and kills Achilles, but doesnt get his power. Her lacking wisdom and courage marks her as obviously evil, and Freddie defeats her by being smart enough to make a kamikaze plan, and brave enough to carry it out. It also helps that Zeus guided Freddie to visit these two humans first, to ensure he was worthy of Shazam.