In this ayaah Allah is telling us a story. The story of a people who were given the book. Allah tells us that they had some sort of relationship with the book they were given. They may have read it beautifully, memorized it, celebrated it etc. but as soon as it came to this book making decisions for them, deciding right and wrong for them, they turned away.
If there is one word which could describe the heart, it would be "change". How many times have we experienced our feelings, our emotions and our inclinations change. Our desires, our wishes, our passions, everything is in a continuous roller coaster of change.