I think it's because Ryo is a man and he can't give birth to a child. The purpose of Aqua (forgot to spell his name) going to Japan was to find a bride to bear Lhutan children as the tribe's population is mostly men. Otherwise the tribe will just die out because there's no reproduction.goibg on. However, I am not sure if this is right
To make it short:
Ryo idealized Lutah's tribe. Making them saints with no defaults, people who are kind, beautiful, generous etc. And after seeing Eigena and learning about its meaning, he started to think he was not virtuous enough.
Because Qaltaqa is supposed to become Lutah's next chief, he should make a baby someday to take his place, that's how Qaltaqa was educated. And for that, he should obviously marry a woman.
Ryo thought then he is not great enough for this land full of "saints" because he's selfish. He thinks that for the tribe's and Qaltaqa's happiness, he should let go of Qaltaqa. Because he'll never be able to get children and the elders rejected him because of Ryo. But he can't bring himself to be apart from Qaltaqa's.
That's what made him feel bad. Now that he talked to Aqnam, je understood the Lutah tribe is no saints, they are differents people with different personalities who can also be mean or feel upset. That's why he decided to stay. Because he has no reason to give up his love, and his selfishness doesn't mean Eigena won't accept him.
It should be accurate I think.

Could someone tell me whats bothering ryo? I don't get it... I'm a slowpoke. ( ̄∇ ̄")