As is my preference I like to start answering a question like this by looking at the history of the word. Psychology is a Greek compound first penned by the Renaissance Humanist Marko Marulic, meaning words concerning the soul. The words concerning part or ology is fairly familiar from any other science you have studied; however the word soul has an interesting history in its own right. Soul is a kind of abbreviation of the Greek word psuke or breath. In its long wanderings from ancient Greek to our very modern and quickly decaying English, psuke lost the p and picked up an l to become soul. However the early Psychoanalysts resurrected psuke and restored it to our vocabulary in their description of the Psyche, however they meant something much more intimate to our internal life than merely breathing. Up until Marulic's time much of what we call psychology was addressed in books titled de Anima and Ethics. De Anima, like psychology is translated into English as on the Soul, however the Latin anima is more closely related to our word animated or animal then is the Greek, which seems to highlight a behavior common to all animate things, namely breathing, rather than to the group of breathing things as the Romans do. Psyche, Anima, and Soul all have a similar foundations signifying the breath of life. Aristotle goes on to define the soul as the principle of life in a thing. Thus anything that is alive has a soul.
“The Soul is the Form of the Body” - Aristotle
In his book on animated things, the de Anima Aristotle treats of three kinds of soul; those of plants, animals, and men. It doesn’t take much observation to note that plants, animals and men all present very different kinds of animation. This is why its offensive to call someone "a beast" "a pig" or to call a coma patient a "vegetable." Life is the ability of a thing to direct itself towards its end, its perfection. What distinguishes plants and animals is the degree to which they participate in obtaining the means necessary for their perfection. Plants are passive in their reception of food and reproduction - after all to say a plant seeks its mate is hilarious - whereas animals are more active in the acquiring the means by which they perpetuate the species. While animals are more active in acquiring the necessities of life, they are however determined to certain particulars. This is why we can make such hard and fast distinctions as "herbivore" or "carnivore." Man, though animal has the added perfection of being rational that is to say, like animals we actively seek out and engage the means of our perfection but we also have to do so not through instinct but by choosing from a variety of possible means. I can be a vegetarian or eat a paleo diet or I could choose not to eat at all. This small example is but a sample of the wide panoply of choices arrayed against us everyday.
Even in these simple choices it is often desirable to receive counsel. Sometimes we become habituated in making certain choices which are no longer effective in making us happy. Now a habit is a firm disposition toward a particular action and therefore often requires the help of an outsider in order to move us towards different action. One of the roles a therapist plays in the life of his clients is to help them experience control over their behavior and to make choices based on reason and not solely on the emotional instinct common to all animals.