"Go out looking for just one thing and that's all you'll find."

This quote means that you need to be open minded. If you are focused on one thing then you'll end up only finding what you're looking for.

It is similar to looking for a certain lego brick. I'll be vigorously looking for a piece that I need in order to finish my ultimate creation. I end up passing by severel hundred other pieces that I don't think I need. I finally find the piece and then relize that the next piece I need has been popping up constantly. I search for that piece but it is no where to be found. Suddenly I see tons of the piece that I was searching for so vigorously.

In short if you are so consumed with one thing then other might pass you by that won't be avaliable later.