Fear

The oldest and strongest emotion of mankind is fear, 

and the oldest and strongest kind of fear is fear of the unknown.

~ William Shakespeare

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I believe that fear of the unknown probably is the strongest kind of fear.  I think that we make up horrific tales in anticipation of events - most of which are unlikely to occur.  We worry about things - "could be" things and "what if" things and "might be" things.  

And worry is a waste of time - though you can successfully do that and do other things at the same time.  I don't believe in multitasking, but I guess you can worry and drive or worry and run or worry and look under the bed simultaneously.  Of course that means you aren't worrying or driving / running / looking under the bed as effectively as you would be if you did those things separately.

Now, I do not believe that fear of the unknown is the oldest fear.  I think the oldest fear is fear of crumbs in pockets or drawers.  I hate that.