David used selective focus to draw the viewer’s eye to the thorns by letting the stems and leaves in the foreground blur and frame the center of interest. The thorns are pretty much centered and the stem goes north south in and out of the frame. That part is a little distracting. I could see cropping this down to just the thorns and you would have a tilted diagonal image. This is a good example of using elements to frame the point of interest.
Another way to gain selective focus is a device on the market called Lensbaby. It is a device that when attached to the camera body will selectively focus. It has many models now and can be quite expensive with the bells and whistles. Then, there is Photoshop, which can also be used.