From the book's back cover:

Every person must have an ultimate aim which is the focus of his love and will. If a person does not have Allah as the focus of his love and will, he will inevitably be enslaved by something else, whether it be his own pride and arrogance, wealth, power or something that he takes as a god instead of Allah. 

The heart cannot be sound, or succeed, or find joy, or be happy, or feel pleasure, or be good, or be at peace, or find tranquility, except by worshiping its lord, loving Him and returning to Him.