"'He causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous.'" NIV translation
Why are we to love our enemies? The simple answer is because God loves them and is allowing them to benefit from His grace just as we do. The time for judging the unrighteous will come but, today, His mercy and grace are still flowing to all.
"'If you love those who love you, what reward will you get? Are not even the tax collectors doing that? And if you greet only your own people, what are you doing more than others? Do not even pagans do that?'" NIV translation
Jesus explains that we are to demonstrate the love of God to all men so that we stand out. The whole purpose of us standing out is so that we have an opportunity to tell people of why we are so different. This is the chance to tell them about Jesus so that they might be saved as well.
"'Be perfect, therefore, as your heavenly Father is perfect.'" NIV translation
As His children, we are to be a good reflection of our Father. The kingdom way is perfection and we will get there when He comes to rule and reign. Until then, we do the best we can through the power of the Holy Spirit.