Why do we need to guard our self and what can we learn from this?

Answer From The Annual Reading Schedule - Week 42 Notes For Deuteronomy: So that we don't forget the words we have seen, so that they don't turn aside from our heart all the days of our life (Deuteronomy 4:9). What we learn from this is that remembering and doing the commandments, laws, right-rulings (judgments), etc. is not some "only-outwardly" thing. It starts with the heart. Some try to teach about the law in a way that implies it is all fleshly and nothing to do with the heart. However, verses like this show that it actually starts with the heart.