Because God is love.  God's law reflects the very nature of God.  If you reject the law, you reject God.

Romans 13:8-10

Owe no man any thing, but to love one another: for he that loveth another hath fulfilled the law.

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.

Love worketh no ill to his neighbour: therefore love is the fulfilling of the law.

Galatians 5:13-14
For, brethren, ye have been called unto liberty; only use not liberty for an occasion to the flesh, but by love serve one another.  For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself.

This ability to obey is one of the many free gifts that come along when you accept Jesus as your Savior.  It is the power of Grace in the life of the believer.  It is the fruit of the Holy Spirit.

And the only way to fulfill the law is by yielding your heart, soul, and mind to the Holy Spirit of Christ.