In my personal opinion I think it needs one of two things:
Something in the center to fill up that gap, filling up the final third OR Just have the picture with the dog(s) on one side side, I've often found (and been told by many people much better than I) that photographs an often look good if there is something on just one side of the frame, but It never works if there is a subject on both sides but nothing in the middle.
Also, I find the extremely bright background and the tree quite distracting, but then that could be due to how there's nothing in the middle so my eye is immediately being drawn to that bright area
As others have said, the tree brings the eye to the right dog; in addition, if it were not for the tree, it would be very hard for your eye to "decide" which dog is the subject; it is better to have one place where the eye is drawn, like moving them closer together as one subject.
Your foreground is off as well as your background. The depth-of-field is also off. If you can see what I see in the middle, the faces of both dogs are off blur, meaning the are blurry. Also the faces are over exposed a touch, there is a good distance between the dogs. I will download this and edit the way I think it should look and show you what I'm talking about.