Holding onto an idea

In the movie The Weatherman, there is a scene where Nicholas Cage's character is walking to the store to pick up food for his family. Prior to leaving, his wife had "asked" (i.e. told) him several times to bring home the sauce she likes. She knows his history of flakiness and absent-mindedness. As he is walking, we understand her perspective. A mental voice-over plays his highly fragmented thoughts: we hear him fighting, fighting, fighting to hold in his mind this idea of "buy the sauce". By the time he arrives at the store, the idea is gone. He forgot. Again.

I think most of us can identify with this - as we've all forgotten something at one time or another. But what is particualrly interesting to me is why it is sometimes so difficult to "hold" onto an idea, and what makes it difficult. How do we get distracted in such a short period of time, when just prior we were sure we were going to remember?!