Many real good things along with bad we do inherit in our animal nature: not only aggressiveness and violence but also “natural” kindness, teamwork and even compassion could be explained as the highest form of animal instinct. The real question is whether the feature is supernatural and, more, why?
Why am I kind to a person? Only because of solidarity (today I help you because tomorrow I may need help from you)? That's just an instinct of self-preservation, some variant. Why I spend so much time and effort my children – to be continued in future? That's the species preservation, biology again. But why love?
Here's the point. Evil is natural and so is kindness. Love is not: it's supernatural. A Heavenly Bonus. You can never say why there is love. Love is not why, love is because. Because there is God's image in a human. Because the Spirit has touched this clay and made it not-just-nature. Because biology really costs not a lot when it takes real Humanity. Especially when it's Christ's.
In either the Scripture or the Tradition God might bring us understanding of all His and Heavenly. All but love. Love is so supernatural that even the greatest of the Prophets and the brightest of the Texts could touch but never bring it to us. It took God alive, Jesus. Because love can not be taught: only shown and lived along, lived through.
Jesus said: go teach. Jesus did: love. Not even did: He does love. Now, here, but even more serious than in those “good old times”: now He, being Himself in Heaven, gave us the Spirit to do through us that supernatural, love.