Beware and guard yourself lest you forget the words that your eyes witnessed [at Sinai] (Deuteronomy 4:9).
While forgetting is a spontaneous occurrence, it is nevertheless perfectly appropriate to instruct someone not to forget. Personal experience is that if we have something extremely important to do and we are afraid we might forget it, we leave ourselves various reminders to make certain that we remember.
Except when it is due to an aberration in the brain, forgetting something is an indication that it was of relatively little importance. How do you feel when someone who you expected would remember you does not know your name? Also, do you not feel awkward upon meeting someone and having to admit you do not remember his/her name? These feelings are due to the awareness that forgetting something indicates that it was not all that important.
The revelation at Sinai at which we received the Torah was not only the most important event in the history of the Jewish nation, but also the event that should be the fulcrum of the life of every individual Jew. It is the Divine origin of the Torah that makes its values permanent and unalterable, rendering it beyond human manipulation. If we forget the Divine origin of Torah, we are likely to tamper with it and adapt it to comply with our own wishes. When this occurs, all values become relative, and this may result in the behavior of the individual and the group being determined by expedience, hardly a standard of ethics that dignifies a human being.