Miyazaki says, Porco Rosso was a silly thing to do: A move for children that cannot be understood by children.
Porco Rosso is a man alone. He was once a flier, with all the manly adventure & romance of flying, especially during the early years, during the war. But he lost a friend & with him, his humanity. He is now a pig. If you think of a pig as yearning for his lost humanity, you will see the romance. But a pig is a comical-looking sort of human being & has some virtues of which he is unaware. For the people who know him, he is a scratched-out face in a photograph with his friends. They are dead, he is still alive.
The story shows the stages of recovery of humanity by coming to agreements with people. First, Porco Rosso makes a deal with some bandits to give up half their loot & the girls they kidnapped. It is not all of justice to allow them to steal, but people need to live on something. Porco Rosso is a bounty hunter now, so he is especially situated to be humane where the laws fail to act.
This brings up the problem of friendship–an American pilot shows up who wants to compete with Porco Rosso, who wants nothing to do with him. This is the structure of the plot: Enmity turns into friendly competition &, finally, cooperation between these two, in stages.
Everything good for Porco Rosso comes from the evil the American does him. Thus does he get the chance to save his life, & thus treasure it, to ask for help, & thus become aware of his need for other people, & therewith of their worth, to make friends, & thus to see the innocence in the striving to fly which he no longer understands, & finally to compete again, in full awareness of the limits of being human, desperate to succeed though he is aware of his mortality. The dogfight finale rehearses all this, giving the key to the structure.
But there are certain surprises. One is that an American, with all his brashness & love of celebrity, turns out to teach Porco Rosso he should take life seriously. Why does it take histrionics? You could say a dashing American is a mirror-caricature of manliness: He looks like what Porco Rosso is, who does not want to show himself. Then there are all the women who do the actual work in technology. Porco Rosso learns to respect their powers, their helpfulness, & their friendship. They, too, mirror his deformation.
These metamorphoses allow Porco Rosso to speak about the beautiful truth about his guilt & his loneliness. He had a vision of the beings beyond heaven, where all the fliers are together, no longer enemies, no longer limited by their bodies. The flier, the motor, & the plane are the soul.