Selective Memory?

I ponder a simple question that came to me yesterday as I lugged up Summit Hill for a continuous hill workout. I know before I'm going to start this workout that it's going to hurt. It hurt the last time I did it. Yet somehow in the 1.43 mile warmup to the hill I forget that there's going to be any pain involved and trot up the massive hill. That is of course until I reach just past where we stop for sprint hills and the intense burning from lactic acid starts to creep in. Then all of the sudden that memory comes back to the previous times of doing this workout. It's quite painful! How do you forget something so vivid so quickly?