I remembered the day I started on my journey to become a pro-Photographer. I was on the phone with one of my future mentors. I asked him a silly question which at the time felt serious and important in my head. I was so impressed at the quality and creativity of his photos compared to what I had been shooting. I asked him "why do professional photographers photos look so good"? After a few seconds of silence he replied "That's just what we do!" I chuckled and realized I had to learn how they did it.
It's been 8 years and I think I've figured some of it out but the best part is the journey never ends.
Here's a great article on that very subject:
http://www.picturecorrect.com/tips/what-makes-some-photographs-better-than-others/