Zenitsu fell in love the minute he laid eyes on you. However Kaigaku was a slow burn.

Zenitsu constantly fanboys and simps for your ass. Kaigaku gets irritated with Zen's simping. They're both obsessed with you, Kaigaku just doesn't show it much.

Kaigaku is the type to be confused about these feelings and annoyed at them, while Zenitsu was familar with this feeling of love, he just didn't know how extreme his love was.