So, let's look again at the gear shift.
'Forward' is the direction towards what we desire. When we see something, and our consciousness through memory assigns a positive attribute to what we see, then the will moves into a forward path towards the object.
We become 'attracted' to what we perceive.
We desire, we want, either to consume or to possess. This forward motion is called by some of the Church Fathers the 'appetitive' quality of the soul, which is the capacity to desire.
Desire moves a man out of inertia. We get out of bed because we desire to move, we eat because we desire food, we work because we desire to be active and to have things. Desires make the world go 'round.
Man cannot be utterly rid of his desires, because otherwise he would not do anything at all. It is foolish to think of a person without desires. But, we must then ask the question: are all desires good?
In a healthy person, the answer is 'yes.' A further refinement of the definition of desire is necessary at this point: desire is always for what is good. The human will seeks what is good. Now, there may be desires for things that are not good for us at a deeper level (for example, the desire for sugar can lead to obesity and tooth decay in inappropriate amounts), but the desire for sugar itself is not bad.
Where humans run into problems is when desires are not met and inappropriate replacements are made. So, for example, we all desire (to varying degrees) human companionship. If we deprive ourselves of this companionship, we will often end up turning to a replacement: food, pornography, drugs, hoarding... the list goes on.
Desire is based on need. The human person is, in reality, needy. But, that is because life itself is a condition of need. Even the most basic organism has needs. Life is about being needy, consuming and eliminating. In and out. We are in a constant flow. This condition of perpetual need is elementary to all life.
Only a fool would try to exist without need.
So, the anticipation of need is a desire: I am attracted to food either because I am hungry or I know that I will be. The mind assigns a 'good' label to food, and when I see it, I am attracted to it. I have a desire for it because I know about hunger and the underlying needs I have.
Once we explain the totality of the gear shift, then we will discuss what happens when our needs become distorted.