Because he didn't sound like he felt bad about what happened. If anything, he was telling his brother that he was glad it happened since he is happy. In other words, he was saying he didn't give a damn about losing him because his excuse was that he was an omega and so he deserved this happiness
I don't know why everybody hates this ┑( ̄Д  ̄)┍ The letters had me in my feels fosho