It is because the initiating impulse for the work comes from the vital. The vital, in most people is like that. It jumps at something but cannot keep a sustained interest. It is better to step back for a few moments, reflect with the intellect to see what is really drawing you towards the work and then take a more deliberate and conscious decision. It is the depth of engagement that keeps a sustained interest in the work.
Affectionately,
Alok Da