It is said that the heart is made out of memories and experiences. Allowing it to grow, memories build the persona and nurture the heart. No two beings can have the same heart.

And if one creature were to contain two hearts, the stronger heart would assimilate the other. However, the stronger persona would remain, having to bear the weight of the memories contained within that heart.

It is obvious who was stronger in this case. A heart engulfed in fear can't experience things and grow. It remains stagnant and dull until it withers away, shrinking into something smaller than a crystal.

However, fear is also something every being needs to experience.