Luca really felt like an idiot, had crossed his mind to confess his feelings to Katherine, but he was sure that if he had been reckless, then everything would have changed between him and Katherine.
Perhaps they would become awkward, or Katherine would avoid him and not let him always be by her side.
Luca didn't want that. He didn't want Katherine to not depend on him again.
All this time, that closeness happened because Katherine always needed him.
He still remembered when Katherine asked for his help to escape from the house of Katherine's vampire family, after they ran away, Katherine wanted to let him go. Luckily he had the courage to say that he would always be with Katherine.
When he failed to heal Javier, Katherine looked angry and disappointed with him. He was very worried that Katherine might leave him, but then again, Katherine still got him to live with her.