Old or New Law?

We've all heard of the greatest commandments right? If you haven't here it is:

Mark 12:29-31 "Jesus answered him, "The first of all the commandments is: 'Hear, O Israel, the Lord our God, the Lord is one.  And you shall love the Lord your God with all your heart, with all your soul, with all your mind, and with all your strength.' This is the first commandment. And the second, like it, is this: 'You shall love your neighbor as yourself.' There is no other commandment greater than these." 

I feel a little silly for not knowing this earlier than today, but here is my confession. I did not know that these commandments came earlier in the Bible than in Matt, Mark, and Luke. I've only ever heard it read out of the new testament and described as the "new" commandments when Jesus came and died for all. Sadly I haven't ever read it on my own to see the truth behind this scripture. I'm sure pastors have said these commandments didn't originate from the New Testament, but I don't remember; that's a part I've omitted from memory. Well today during my quiet time I read such verses (but in Luke) about the greatest commandments. And low and behold I look down to the footnotes and see two verses that go along with the one that are located in the Old Testament. So I look them up to see what they are.

Leviticus 19:18 "You shall not take vengeance, now bear any grudge against the children of your people, BUT YOU SHALL LOVE YOUR NEIGHBOR AS YOURSELF: I am the Lord."

Deuteronomy 6:5 "You shall love the Lord your God with all your heart, with all your soul, and with all your strength."

Who knew they were first in the Old Testament as part of the original law!? Right after the Ten Commandments were written, the Law was written by Moses and I think some others. Here it is, the greatest commandment (which it is called in the original law as well) spoken long before Jesus ever came into human form. Quite honestly it blows my mind that I didn't see this earlier. And then it makes me think: Did people forget that part of the Law and is that why Jesus brings it back up? Who knows? Only Him.