In the story of Jason's early years, we see that he has all the makings of a great hero. He's trained by Chiron, the wise centaur, who's responsible for teaching lots of other big time heroes (like Achilles). We also see that Jason can be kind, like when he decides to help an old lady cross a river. This turns out to be a really good move on his part, because the old crone is Hera in disguise. By helping a seemingly helpless old lady, Jason gains a patron goddess, which is another must if you're going to be a hero. The fact that Hera is the goddess who helps Jason out is pretty crazy when you look at the story of Heracles, who is hounded his entire life by Hera. Usually, Athena is the patron goddess of heroes, so Jason gets an A for originality.
Probably the most important thing that Jason finds in his early years is a quest to go on. When Pelias orders Jason to go find the Golden Fleece, he gives his nephew the final ingredient in the hero-making formula. Every hero needs some big impossible thing that he has to go off and do. Theseus slays the Minotaur, Perseus beheads Medusa, Heracles performs his twelve impossible labors... the list goes on. Basically, until you travel to some distant land and slay a monster or two, you're just another young punk with some sandals and a sword. So, like so many wicked kings before him, Pelias ironically makes his nephew a star when he assigns the young man a quest that's designed to kill him.