Sketching vs detail-drawing
The image seen with the minds eye is better translated to the real world by using a sketch than an attempt at a detail drawing.
Once you start making out details without having sketched out the main lines, you lose track of the original vision.
The focus on details end up changing the vision, and it is difficult to go back to its original state for an overview.
It is also often the case when converting a sketch into a more final thing, that it looses some of its inherent vibrancy. Something about cleaning up the lines makes it feel less energetic, and my tolerance for faults are much lower.
See sketches.