That is the difference between the Old Testament and the New Testament. The Old Testament was written before Christ was born...it was written to prophesy his coming. There are so many prophesies that had to be met unconditionally that it was the only way to convince the people of that time that Jesus really was the Messiah. The New Testament is referred to as the New Covenant, to be used by us today. It takes some of the concrete, black and white, period specific rules from the Old Testament and gives them a broader, more relationally-centered meaning. And as for the question why am I supposed to love God....nobody says that you absolutely have to love God. A person's love for God builds as they;
1. Give up control and put their faith in God,
2. Learn what God has done for us in spite of ourselves,
3. Strive to know Him more.