Her voice is so pained and hurt and Mettaton knows he's messed up more than he ever has before. As she talks it dawns on him. Who is he to stand here and try to make excuses? He knows this was for curiosities sake. And he knows how it feels to be on the other side of this equation. After all, didn't Frisk do this to him? Didn't Frisk do this exact same thing by breaking into his house and reading his diaries? Remember that devastation, Mettaton? Remember how awful that was?

He looks to Mikan, sees the tears flowing down her face, and knows he can't walk away from the truth of this. He can't walk away from how much he's hurt her in this moment. But, at the very least, he can tell her the truth.

"It's not all I care about, no. But it is a great part of what I care about. Like I told you, I'm selfish. I'm a selfish person. Coming to Wonderland, I have been trying to change. I have been trying to be better. I've been trying to be more considerate of other people's thoughts and feelings. But... clearly here, I failed."

He can't look away from that truth. But her next questions make him respond immediately.

"No! I like you! I love you like you're my family! You've always believed the best in me and looked out for me and comforted me and done so much despite how much you dislike yourself and that has always been so admirable about you. I care about you so much, Mikan."

Wonderland's magic pushes. He hates that it pushes so much.

"I can't even say that caring about you that much would change how I acted. I've broken promises to my family before. I should be a better person to those I care about, but I'm not. It was a stupid, impulsive act, but I do like you. You have every right to be mad."

He doubts she'll ever forgive him for this. His soul twists at the thought but... well. He deserves this.