Shigaraki had you as his first ever partner. So he had absolutely no idea how to act, if he should be sweeter, stay the same or what. You always told him to just act like himself, so he did! He is now a horrible, evil, abusive, scary and Yandere Boyfriend, but he still loves you, when you are perfect and act perfectly without complaints. He isn't afraid to use his decay on anyone or anything, including you.