Based on the charge that Christ gave us, we are to treat everyone equally. We should not have more compassion for our best friend than we do for those who oppose us.
We should be heaping love, peace, patience, kindness, goodness, gentleness and self-control on everyone. EVERYONE.
If you only love those who love you what does that gain you? But, love an enemy and you might just gain a friend.
Another way to look at it is this, Loving those who love us does not take much effort. There is no growth without effort. Love those who are our enemies goes against the grain of our natural selves. We place ourselves in a strange place where we give the other person control of the relationship. They have the choice to accept or decline our love. We should never stop giving it though.
Who in your life should you be giving the control of your relationship?