All we like sheep have gone astray; we have turned every one to his own way; and the Lord hath laid on him the iniquity of us all.

And these words, which I command thee this day, shall be in thine heart:
And thou shalt bind them for a sign upon thine hand, and they shall be as frontlets between thine eyes.
And thou shalt write them upon the posts of thy house, and on thy gates.

Therefore, I will always remind you about these things—even though you already know them and are standing firm in the truth you have been taught.

COMMENTARY

All we like sheep have gone astray; we have turned every one to his own way
I do believe that when most of us make a commitment, we really do intend to hold fast to it. People do not get married intending for it to end in divorce, or sign a contract intending to breach it, or enter baptism intending to return to their old sins. And yet they do. All the time. We are a wayward flock and always have been.
There is a very simple reason for this behavior. We are temporal beings constrained by time. Though we have the memory of the past and the hope of the future, we can only ever live in the present. Memories and hopes can influence us, but the immediacy of now will often drown them out. Thus we may have felt very strongly back when we made marriage vows, signed contracts, and entered baptism; but the weight of those moments do not last forever. Our every vow comes with an expiration date.
If you have noticed this tendency to wander, know that it is not a failing in you as an individual. It is a universal, and unavoidable, failing of the entire human race. We may not forget what we have promised, but we do forget the feelings we held when we made them.

And these word shall be bound for a sign upon thine hand, and they shall be as frontlets between thine eyes. And thou shalt write them upon the posts of thy house.
Therefore, I will always remind you about these things
God is well aware of this forgetful nature in us, and as such has always taught his people to regularly refresh their vows. He instructed the Israelites to bind the words of the laws on their hands, in little boxes on their foreheads, and on the posts of their doors. He had them do this so that they would see constant reminders of what they had been called to. The disciples of Jesus were given the Sacrament, and told to come together oft to repeat that commitment ceremony.
Yes, our hearts stray, but we can choose every day to recommit them. Yes, our vows come with expiration dates, but we can extend them with new ones. When I keep the commandments today, it is not because of a commitment I made once years ago, it is because of the one I made last Sunday when I partook of the sacrament, and this morning when I said my prayers. We are a forgetful race, but we can remind ourselves.