Wait, let me rephrase that. "Explain" is far too ambiguous. It suggests sitting them down and telling them at length why such a thing as forgetting your favorite ice cream flavor really reflects a deeper ambivalence towards you that makes you question his interest and you certainly hope it won't happen again. No. Explaining is a one sentence thing that happens the moment the unforgivable occurrence happens. Do not steam. Do not dwell. Do not ask him if he did it because he's stopped liking you. "Don't do that--it makes me cranky," should suffice.

Now, boys, all of the above having been said: you should know already that she likes chocolate chip cookie dough not chocolate chip mint! Granted, women tend to overthink things, but that doesn't mean you get to stop thinking just to even the balance. (In the extreme, there are these boys whom I envision are something like cows--seeing shapes and colors and little else, "Hmmm, green. Grass. Green grass, good. Girl. Girl upset. Girl upset, bad.") You may think women's motives are unfathomable. They're not, but if you want to believe that you can. But, the thing is that no matter the motives, the actions are pretty consistent. You don't have to understand why she moves--just take the time to understand how.