Scripture sometimes describes God in terms appropriate to humans physically, which is anthropomorphism (Deuteronomy 5:15) or mentally, which is anthropopathism (Judges 10:16). Such passages are usually interpreted figuratively, as describing God in terms that belong to humanity: thus God’s power can be described as His outstretched arm, and renewed activity on behalf of His people can be described as Him remembering (Genesis 8:1). (I should also point out for the sake of clarity that this use of anthropomorphism takes it as a particular kind of metaphor: the word is sometimes used in different ways as well.)
Why not just take them literally? Is it just that it offends our sensibilities to think of God forgetting? The rationale for seeing this figure in the Bible is that Scripture itself forces us to adopt this procedure. Consider the following verses:
Psalm 78:65
Then the Lord awaked as one out of sleep, and like a mighty man that shouteth by reason of wine.
Psalm 121:4
Behold, he that keepeth Israel shall neither slumber nor sleep.
Here we have awaking from sleep and not sleeping both predicated of God. They can’t both be true, at the same time, and in the same way: that would be contradictory. So we interpret God waking up as being a description of a change in His actions, similar to the change in actions of a man who wakes up, while understanding that God never sleeps.
Why not the other way around? Because only one way makes sense: if we take the descriptions of God as sleeping, forgetting, etc., as figurative, we can understand the statements which say that He does none of them; but if we turn it around, we can make sense of neither. Negating a figure doesn’t actually convey any information at all: however, when a positive comparison is made between some aspect of man and God it makes sense to qualify that comparison by denying the imperfection in the analogy. This causes us to raise our mind above unworthy conceptions of God, while retaining the positive data from the figure. And so we can see that God’s actions changed, while not thinking that He actually fell asleep.
This is also what we must do in the times when repentance is attributed to God, since it is also denied that God repents (Genesis 6:5-7; Numbers 23:19; 1 Samuel 15:10,11,28,29,35). We can see that His treatment of men changes, while there is no change in His will.
The logical next step is to extend the principle. So when we see that things of a certain kind are attributed to God only in a figurative way (because those things are also said not to belong to God, which can’t be figurative), the result is that we understand that all the things in that category are attributed to God only figuratively. Scripture itself has led us to conclude that God isn’t grieved as we understand grief, but that His procedure changes in much the same way as ours does when we are grieved.