While much of that is designed in the writing of the character, it also requires the right actor to pull it off.
If Mark Hamill had been a different sort of actor, if he had actually been a 6′ 2″ perfectly fit specimen, no script in the world would have made him feel like the same character that we actually got.
While that feeling that Luke Skywalker was the most approachable character probably hasn’t been a conscious thought from most of the young children who have watched Star Wars over the past 40 years, it probably is something that has been subconsciously understood.