1. Black dragon arc: It was not his cousin it was a translation error. Read translator's notes before running your mouth. He was looking for his lover after being reincarnated 100 years later because somehow he is born with the memories of his past life. There was no development because there is no place for development.
2. White dragon arc: Man it's beautiful. The myth of "White dragon looking for a human that has special powers" is a rumour created by humans but the real reason was because everyone the white dragon found him he was either on the verge of dying or he was already old. So when he found his reincarnation he immediately took him and slept with him.
The stories are tied to mythology. It's very easy if you are familiar with east Asian mythological creatures and stories. Also these kind of stories are mostly based on folktales so culture is also very importan
Koisuru Ryuu no Shima