Reactions are sort of like a reflex; like when you put your hand on the stove and jerk away when it's hot. There are no "thoughts" involved. Quick and easy.
Responding requires thought. And that takes time and effort. Or what some call "consciousness." Getting back to the stove, it's *actually* only hot 10% of the time; the rest of the time it's harmless. 
Emotions work in a similar way; our self-preservation instinct makes us react, but reacting only makes sense a very small part of the time. But responding is a *learned* thing for most people.