Compelling goals are easier to achieve because they are positive. For one thing the brain processes positive information faster than negative information, which probably explains why double negatives are so confusing. It is easier to get moving when the picture of you in the goal-state is compelling. Then the goal draws you toward it, like a magnet does. Thinking of your goal brings its attractive or repulsive properties to mind.
Attractive images attract.
They are compelling and draw you to them. Compelling goals kick in seeking motivation. This kicks in avoidance motivation. Imaging what you will be doing when you have lost weight can discover a positive statement. Seeing yourself looking good in a bathing suit is more compelling, hence more motivating, than imaging not eating sweets.