The subject matter is the inhumanity of Amalek. To remember what he did, that we understand. Not to forget – what does that add to the discussion?
In fact there are two separate duties. To remember is to make a conscious choice to keep something alive in our memories. Not to forget is to prevent something slipping away into the recesses of our conscience and then disappearing altogether.
There are times when we deliberately want to forget. Some people, for example, remember only the harm others have done and intentionally forget the good times and good deeds. Some work very hard to forget their humble origins and eventually persuade themselves that a less than upmarket beginning never ever existed. So it can be important to be told, “Do not forget!”
But such is the dilemma of remembering and not forgetting that there are also times when we do need to be able to forget, especially a slight or an injury. It harms us over and over again when we obsess about something which needs to be allowed to recede and die.
What tells us when “Do not forget” applies, or “Do forget”? Only a mature, sensible conscience, trained in the Torah way of thinking.