An Investor's Greatest Enemy?

[Myles Wisdom]

I was reading a short article today that said something that I found interesting: "an investor's greatest enemy is not really the markets or lack of resources, but really the investor himself". 

I do believe that those things (ie. lack of resources) can actually be real and serious reasons to impede progress but according to the quote, the bigger enemy is ourselves. I'm wondering what is it that makes us the biggest enemy? Is it simply fear or is there more to it than that?..