In this the love of God was made manifest among us, that God sent his only Son into the world, so that we might live through him.  In this is love, not that we have loved God but that he loved us and sent his Son to be the propitiation for our sins.  Beloved, if God so loved us, we also ought to love one another.
(1 John 4:9-11 ESV)
If you do an internet search for "What is love," your results will be filled with all kinds of philosophical pondering. You'll read statements that sound profound, but ultimately have no meaning. Everyone has an opinion about love, from actors to authors, from scientists to scholars.
Currently, a lot of research is going into trying to understand how the brain creates feelings of emotion. Scientists are trying to see what chemical and neurological interactions create the emotion we call love. Their goal is to try and remove the mystery of love and show that it's nothing more than an evolved neurological response to a specific set of external observations. Their view of love will make marriage proposals just a little less romantic, "My neurons consistently fire when I'm around you, therefore it seems logical that I should spend the rest of my life with you. Will you marry me?" That doesn't sound like love, does it?
When left to ourselves, humans will come up with all kinds of definitions of love: scientific ones, selfish ones, and philosophical ones. But, in Christ, God has shown us what love is.
1 John 4:9 says "In this the love of God was made manifest among us, that God sent his only Son into the world, so that we might live through him." The love of God was shown to us by sending Jesus to dwell among us. He loved us by sending his son to be the propitiation for our sins. (Propitiation means that he satisfied the wrath of God in our place). And, he did this not in response to our great love for him, but simply because he is love. Romans 5:10 says that God did this even while we were his enemies.
God defines love for us. 1 John 4:11 says that if God loves us this way then we ought to love one another in the same way. Therefore, love is more than an neurological evolutionary response that just happens based on certain circumstances. When we are called to love our enemy or love those who don't love us or love the unlovely, we're not acting on chemical reactions. We are demonstrating the love God has already shown us in Christ to the watching world.
When Christ arrived as a baby, God showed us what sacrificial love really is. He showed us that love is more than just an emotional response; it's a willingness to love those who don't deserve to be loved.