THE NUMBER 21 IS HAUNTING ME!!!!!!!!!!!!?

This is called the confirmation effect - you see 21 everywhere because you're looking for it. If you wrote down every number you see all day (phone numbers, prices, license plates, times), well, you probably wouldn't have time to do anything else, but you'd see there's nothing special about 21. You're just remembering when you see it and forgetting when you don't - the confirmation effect. Looking for patterns is a very human thing to do, and it gives us a lot of 'false positives' like this - which is why so many people think they have 'lucky numbers'.