Why do we look without seeing?

How many times have you been looking right at an object and not seen it? Wives tell me that husbands are the worst at this. See if this conversation sounds familiar?
Setting: The wife is eating popcorn while watching a movie.
Wife: Honey, can you please get me the salt?
Husband: Sure, where is it?
Wife: Right there on the counter, next to the fridge.
Husband: Ok, I’m standing in front of the counter, right next to the fridge. It’s not here.
Wife: Yes, it is. I put it there this afternoon.
Husband: No, it’s not.
Wife: Yes, it is!
Husband: Nooooooo…………… it’s not!
This dialogue continues for a few minutes until the wife gets up off the couch, marches to the counter and grabs the salt, which is right in front of him. Has anything like this ever happened to you? It happens to me all the time.