Our limited awareness Everything around us is competing for our attention at all times. Things that are large, have bright colors and move quickly usually take up most of our attention. This is because we, as humans, can only perceive a set amount of things at a time. Basically, we only have a bit of attention and we fill it up really fast.
We know a lot more about attention then most people think, read up on cognitive psychology if your interested in more details. In short, attention is limited and has a set of rules for what is more important to be aware of. Most of these rules we learn as we grow (speeding cars will hurt you if they hit you) some are based on instincts (sharp or slithering animals are not to be trifled with).
Thankfully we have cognitive models to help use make the most of our attention.
What are cognitive models? I'll get into that next.