KJV
Mat 5:45 That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.
The Lord our God never to my recollection in scripture teach or tell the Children of Israel to “love your enemy” in the OT. There is “thou shalt love thy neighbour as thyself” but no love your enemy.
So why now that Jesus has come into the world is He teaching us who know Him, to love your enemy?
Well, i'm not sure that I understand your question as the answer is quite clear, Jesus was contrasting former OT principles, with their true and greater meaning.
i.e. Don't commit adultery, ...but rather, don't even lust at a married women.
Observe the Sabbath, ...but not at the cost of one's well being.
Don't murder, ...but more so, do not even hate.
Do not break a vow, ...instead, don't make a vow.
Again, he even says further down the sermon, that loving one's enemy surpasses that of those who are evil, and only love those who love them (there is no sacrifice in this relationship, their love is reciprocated) . God's children do as God does, that is, we hate God, yet he loved us. Christians sacrifice themselves for others, i.e. we don't expect payment when we lend, we don't invite those who can only invite us back, when we see someone down, we lift up without expecting a reward, and thus, we love those who do not love us back.
But, don't be naive, know what you're doing when you do this, that is, know why Jesus said it. Because if you love those who spit on you, and abuse you, and you don't handle it with confidence and wisdom, it will have an adverse affect. Just like turning the other cheek. These are not principles for bleeding hearts or the naive. They are profound, and meant to be executed with the utmost wisdom and understanding. Again, the wrong disposition when attempting these maxims, will be consequential to both parties, and not beneficial as intended.