the excitement of waiting

Why do we tend to be more anxious when we wait for something?
We realize how much time passed by, when we are sooo excited to achieve and do something.
Why can’t be waiting, be just like any normal activity?
Why can’t we rationalize?  Or is it because we take many things into consideration that makes waiting all the more exciting?
What is in it that makes the feeling intense?  Is it the outcome? The action to be taken? Or is it more of fear of the execution of your action?