Two teenagers, the same father doesn't love them, and they don't have a mother. The sons of the Tian family are just pawns in the hands of the regent. The third prince of the Huns is not trusted by the family because of his impure blood. This seems very silly, but luckily they met, got to know each other, and understood each other. We can support each other even if we are not taken seriously.
But after all, what Chu Su had in mind was not that simple. If he wanted to make the Xiongnu surrender to Dayan, he had to make the future Xiongnu king "submit" to him first. Since ancient times, this has been the principle of capturing the thief first before capturing the king.