Reasons to attack the middle
I did some video coaching for someone recently. One of my primary comments was that over and over his first loop went to the corners, where the opponent was ready. Instead, I recommended his first attack primarily should go to the middle (i.e. roughly at the elbow, the transition point between forehand and backhand). Why? It's much harder to block or counter-attack from there, as 1) the player has to decide whether to play forehand or backhand; 2) he then has to move into position, which is usually harder than moving to cover the corners; and 3) it draws the player out of position, allowing you to attack to the open corner, or (if the player rushes to cover it), to the other corner, or right back at the middle again.
Far too often players attack the corners with the idea they are looking for a ball to attack to the middle, with the common result of a strong return that they can't attack effectively. This is backwards - instead, attack the middle first, and then look for a chance to attack the next ball to the corners or the middle again.
Personally, I love opponents who mostly attack first to the corners, making my life easier. I'll buy my peers a drink if they promise to do so at key points. I hate with a vengeance those who attack my middle, who simply do not understand the "Do not go here!" sign implied by my constantly missing against those shots.