This is what director Miyasaki says:
Ashitaka is at a loss as he comes into the outside world, that is,
town, from his village. At this point, he is hiding his face to show
that he is a non-person. Actually, at the moment he cut his topknot
off, he was no longer human. Cutting one's topknot in a village has
that meaning. So, it looks like Ashitaka leaves (the village) of his
own will, but actually, the village forces him to leave, I think.
Ashitaka, as such a boy, cannot negotiate well when he goes to the
market. The Northeast area, where Ashitaka's village was, used to
produce gold. So Ashitaka just offered a gold grain instead of money,
not knowing the value of it.
- How about going back to the Emishi's village?
M: He can't go back. Even if he could go back, what would be there?
There might be some time lag, but eventually, the world of what Eboshi
has been doing at Tatara Ba will come rushing in. So if Ashitaka says
"I will go home" since his curse was cured, that will be no solution.
And it will be a big problem if he brings San back.
- Kaya, who saw Ashitaka off, loved Ashitaka, didn't she?
M: Yes of course. She calls him "Anisama (older brother)", but it just
means that he is an older boy in her clan.
- So they are not real brother and sister.
M: If they were, that wouldn't be interesting at all. There used to be
a lot of marriage among blood relations in Japan. I thought of Kaya as
a girl who is determined to do so (marry Ashitaka). But Ashitaka chose
San. It's not strange at all to live with San, who lives with such a
brutal fate. That's life.
Source:Miyazaki on Mononoke-hime