“You shall love your neighbor as yourself” is the second most important commandment in the Bible, even the first commandment depends on the second. What is the first commandment? “Love the Lord your God with all your heart”. Why does the former depend on the latter? Well, John says: “How can you say that you love God whom you have not seen and not love your neighbor whom you do see?
The Bible says in Romans 13:7-9 (KJV):
7 Render therefore to all their dues: tribute to whom tribute is due; custom to whom custom; fear to whom fear; honour to whom honour.
8 Owe no man any thing, but to love one another: for he that loveth another hath fulfilled the law.
9 For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself.
The apostle here details a series of commandments that have to do with the moral law and in the end he says that all are summed up in loving our neighbor as ourselves.
Can you say that you love your neighbor if you steal or bear false witness against him? In no way, then, if you really love your neighbor, all the other commandments of moral law will be like a piece of cake.
A true Christian will seek to fulfill this great commandment, because in this way he will honor God and His word and show that he is a true Christian who loves God.