Humans are naturally what scientists call cognitive misers.
Meaning that we naturally have the desire not to think a lot if we don't have to - that there is only so many units of energy in my brain today and that I can't possibly focus on everything that is thrown out at me.
So we filter things out.
Things that require an excessive cognitive load we ignore - because its just to much work.
What's the cognitive load of your marketing or website? Is it easy to identify whats going on?
If there is to much cognitive load I'll leave to something easier - if there is to little I won't be stimulated.