What does a “good” person have in common with a “good” cheeseburger or a “good” story? You don’t think a cheeseburger or a story are good because they’re honest and caring. Again I’m not being facetious. Thinking deeply about this is what will ultimately help lead to true happiness.

Let’s change adjectives. Some people are naturally faster than others. Fast being the characterization of being quick relative to the average of a particular category or instance. If I want to be a faster runner, wouldn’t I first need to know what fast means? As in, why would I spend my time bulking up and gaining weight like a sumo wrestler if my goal is to be fast? Knowing what “fast” means (in this case “good”) is of utmost importance.

Back to the context above. Like you said, some people have a disposition of being more good (faster) than others. But if I wanted to be even faster than I am naturally, wouldn’t I benefit from training on my own? So too we can become better people using our reason without the need for divine revelation (just as people like Socrates, Plotinus, and the multitude of people that are still good people without Christianity). But what if I could ask someone like Usain Bolt to teach me how to be faster? That’s the case with Jesus (God) who is perfectly (meaning the absolute of the abstract concept) good. He established his training academy (the Church) with approved teachers (the Magisterium) to guard and promulgate his divine revelation (Sacred Tradition and Sacred Scripture) so that all may know how to be better people (or faster like the analogy). What better way to learn to be good than from the fastest there is, was, and ever will be? Sure a gym teacher (non-clergy) who’s never been taught by or even heard of Usain Bolt could still teach me how to be faster but it would only take me so far.

If someone has a trainer to be faster (like Usain himself), would you say they’re not a fast person? If someone looks to Jesus for his help in being a better person, are they not really a good person?

This analogy has some flaws like treating God as an option or a utility (means to an end). He’s not an option and we are to love Him for Him not for oneself. However, it should hopefully make apparent the point of needing to know what “good” means in the abstract just as we know what “fast”, “big”, “hard”, and other adjectives mean.