That "ancient law" sounds like awfully contrived BS... but okay, let's see her reason. She's feeling guilty over the carnage she wrought to avenge being betrayed by humans, so she effectively imprisoned herself. Then she's lonely (perfectly understandable, god knows how long she cooped up in there); and the law says she must now practically throw herself at the protagonist one who helped her, her choices being swear allegiance or be a mate -> she chose the latter because she was lonely and probably wanted the affection of romantic love.
The law part is amusingly close to what I said in jest last chapter, but I won't deny that she's at least charming and cute. I think I might've appreciated it more if her reason was like "I swore to only take someone stronger than me as my mate, and now that you won, you fit the requirement." rather than some "law" that I don't even know who will enforce since she's supposed to be the ruler of the dragons. But really, even just "I'm lonely" is perhaps enough as a reason without the need to follow some contrived BS law.
I like that there are plausible reasons why the protagonist is largely devoid of personality: could be the horrific surgery, PTSD at having been used as a killing machine or even something that haunts him from the past as a flashback teases. But please develop him later from that.