Arata holding himself back because of he's finally being honest (at least internally) that he loves Hishiro. But also to add that he cares about his friends so much that he doesn't want them to have missing memories has him constantly underestimate his worth and value towards them, it's so heartbreaking to see
Like Kariu said, if Arata wasn't there, the whole situation would only been in disarray because Chizuru wouldn't have been able to recognize her flaws and would be too set on her ways. It would have been too late by the time she realizes it if it does happen.