Objection 1. It seems that the will is moved, of necessity, by its object. For the object of the will is compared to the will as mover to movable, as stated in De Anima iii, 10. But a mover, if it be sufficient, moves the movable of necessity. Therefore the will can be moved of necessity by its object.
Objection 2. Further, just as the will is an immaterial power, so is the intellect: and both powers are ordained to a universal object, as stated above (01, ad 3). But the intellect is moved, of necessity, by its object: therefore the will also, by its object.
Objection 3. Further, whatever one wills, is either the end, or something ordained to an end. But, seemingly, one wills an end necessarily: because it is like the principle in speculative matters, to which principle one assents of necessity. Now the end is the reason for willing the means; and so it seems that we will the means also necessarily. Therefore the will is moved of necessity by its object.
On the contrary, The rational powers, according to the Philosopher (Metaph. ix, 2) are directed to opposites. But the will is a rational power, since it is in the reason, as stated in De Anima iii, 9. Therefore the will is directed to opposites. Therefore it is not moved, of necessity, to either of the opposites.
I answer that, The will is moved in two ways: first, as to the exercise of its act; secondly, as to the specification of its act, derived from the object. As to the first way, no object moves the will necessarily, for no matter what the object be, it is in man's power not to think of it, and consequently not to will it actually. But as to the second manner of motion, the will is moved by one object necessarily, by another not. For in the movement of a power by its object, we must consider under what aspect the object moves the power. For the visible moves the sight, under the aspect of color actually visible. Wherefore if color be offered to the sight, it moves the sight necessarily: unless one turns one's eyes away; which belongs to the exercise of the act. But if the sight were confronted with something not in all respects colored actually, but only so in some respects, and in other respects not, the sight would not of necessity see such an object: for it might look at that part of the object which is not actually colored, and thus it would not see it. Now just as the actually colored is the object of sight, so is good the object of the will. Wherefore if the will be offered an object which is good universally and from every point of view, the will tends to it of necessity, if it wills anything at all; since it cannot will the opposite. If, on the other hand, the will is offered an object that is not good from every point of view, it will not tend to it of necessity. And since lack of any good whatever, is a non-good, consequently, that good alone which is perfect and lacking in nothing, is such a good that the will cannot not-will it: and this is Happiness. Whereas any other particular goods, in so far as they are lacking in some good, can be regarded as non-goods: and from this point of view, they can be set aside or approved by the will, which can tend to one and the same thing from various points of view.
Reply to Objection 1. The sufficient mover of a power is none but that object that in every respect presents the aspect of the mover of that power. If, on the other hand, it is lacking in any respect, it will not move of necessity, as stated above.
Reply to Objection 2. The intellect is moved, of necessity, by an object which is such as to be always and necessarily true: but not by that which may be either true or false--viz. by that which is contingent: as we have said of the good.
Reply to Objection 3. The last end moves the will necessarily, because it is the perfect good. In like manner whatever is ordained to that end, and without which the end cannot be attained, such as "to be" and "to live," and the like. But other things without which the end can be gained, are not necessarily willed by one who wills the end: just as he who assents to the principle, does not necessarily assent to the conclusions, without which the principles can still be true.
Source: Summa Theologica (New Advent)