Why do we need habits? If what makes people different from animals is their ability to know truth as such, wherever it is to be found, and not only some limited environment, and again, their capacity to love and seek for goodness, wherever it is to be found, habituation seems to be a kind of degradation of the human being. To act from habit is like acting from instinct, rather than from judgment and choice, insight and love.
To this I would say that it might, indeed, in some sense be ideal if we could have complete insight into the situations in which we live, know all of the implications and effects on human goods and relationships that various possible actions would have, and in view of all this, make a conscious choice before doing any new action.
All this, however, is mere fantasy, or at least speculation (maybe Adam could actually have made decisions that way before the fall). It is totally impossible for us to have every single aspect of a situation in mind, and to explicitly trace all of the particular goods involved in our actions (health, pleasure, the good of laughter, relationship with friends, material goods, etc.) in a detailed manner back to the supreme good, God himself. If we were to attempt to do so, we would take a ridiculous length of time to decide, for instance, when shopping, whether to buy a cheaper but slightly less good tasting or less healthy food or a more expensive but better food.
In order to make decisions in a reasonable time frame and to act upon our decisions, we need to be immediately inclined to many particular goods to be loved and pursued: to the good of our family and friends, the good of society, the good of studying, the good of music, etc. Moreover, we need to be able to easily and readily make a judgment of the proportion of one good to another, as in the example of spending a little more money for better food.
This "readiness" and inclination to pursue certain goods, to favor one over another, to make a certain balance between them, is the result of a conditioning that occurs over time, as the result of many particular actions and we do, reactions we have to events that happen, our favorable or unfavorable perceptions of other person's behavior, especially those we consciously or unconsciously see as role models. When this conditioning acquires a kind of stability, it is what Aristotle and St. Thomas call a "habit."
In English we reserve the name "habit" for the kinds of conditioning that lead us regularly to a particular action, even without thinking about it. We don't give the name "habit" to primarily perceptive capacities, such as an acquired ability to readily recognize the music of Chopin, or to the acquired capability of quickly weighing up the advantages and disadvantages of one purchase or another. The name "habit" is mostly restricted to inclinations to actually perform a certain action. And even then, we usually don't use the term "habit" to the disposition to perform a certain action if a conscious decision is involved. E.g., if a husband has acquired the readiness to stop watching television, or to take out the trash, or other such things, as soon as his wife asks, we wouldn't tend the use the term "habit." But essentially the same kind of conditioning has occurred that occurs when we get into the habit of pressing the snooze on our alarm clock in the morning, of ruffling our hair, of arriving early (or late), etc. The main difference is that some of these habits (or dispositions) involve mostly our reasoning or perceptive faculties, others involve our instincts, others involve the whole person, with thought, choice, and emotion. We speak of habits mostly in reference to those dispositions that involve instinctive or even mostly unconscious action.
One disadvantage of this usage in speech is that we may not recognize all the parallels between this sort of conditioning, and the conditioning of our person–thought processes, decision making processes, and emotional reactions–that is essential for virtuous behavior.