Why do you never notice your own mistakes?

I was devastated today to have two small typos pointed out to me in my new book. This is after it has been proof-read by 4 colleagues/friends and almost two dozen copies sold in the past few days at work with nobody noticing. Once you actually know that a mistake is there, you see nothing else. I will be submitting a revision to the printer this evening but it has really put a big downer on my day and on my pleasure at see my book finally in print.