I think... Ayu is the one who wants to focus on her career and all that. Nagase probably wants to settle down, and they have conflicting thoughts and interests from there.
Seriously, I don't think any of them is to be blamed, especially when there's no cheating or whatever. They have conflicting interests, so they separated... oh well.
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YAYAYAYA GAGAGAGA DADADADA WOWOWOWO
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