I committed myself to read all the texts for our medieval comprehensive exam that I haven't already read so I can be a minimally competent grader. The reading will probably be giving rise to various posts (already hss).
Aquinas argues that our beatitude can only consist in God. The argument is interesting:
the object of the will, which is man's appetite, is the universal good, just as the object of the intellect is universal good. From this it is clear that nothing can put man's will to rest except the universal good. But the universal good is found only in God and not in any created good, since every creature has participated goodness. Hence, only God can satisfy man's will....
An initial worry is that the argument rests on an equivocation in "universal". The object of the will is the universal good in the sense that every good that a human can have is an object of the will. And God is the universal good in the sense that all goods are goods by participation in God.
Here is a more charitable take on the argument—whether it's what the angelic doctor means, I don't know. Every created good that a human can have is a good we desire. We cannot have them all, however. For instance, no matter how many friends we have, we could wish for people with a new configuration of characteristics to be friends with. Even if over the course of eternity we were to be friends with all possible kinds of friends, we couldn't simultaneously be friends with an infinite number of people. At any given time, we can only enjoy a finite number of goods. Now, in this life, we sometimes feel ourselves satisfied by a single created good—say, when we are engrossed in a wonderful conversation. But this is due to our lack of sensitivity, due to the fact that the presence of the good blocks out the fact that we lack other goods. True beatitude is not built on lack of sensitivity. Moreover, a component of a purely created happiness will be a commitment, for a length of time, to a form of activity, and such commitments can be incompatible, with there being a kind of sorrow that one is not simultaneously engaging in others.
If we are sensitive, we appreciate every created good we are capable of having, but we cannot have them all. So, if we are limited to created goods, our happiness will always either be blind or have a sorrow. Limited to created goods, our will is insatiable. But if it is possible to possess that by virtue of participation in which all the created goods are good, then by possessing that one being, one would satisfy the universality in our will. If we have that by participation in which friendship with Albert Einstein is valuable, then we do not need friendship with Einstein to satisfy the will. Thus, one kind of universality in our will—the universality of every—is satisfied by the other kind of universality.