R. Shalom Carmy, "Tell Them I've Had a Good Enough Life" in R. Shalom Carmy ed., Jewish Perspectives on the Experience of Suffering, pp. 107-111 (available online here):
What is the source of man's perennial optimisim? One possibility is that we consider the good of the world to outweigh the bad because our survey of the world has demonstrated this to be the case. According to the Rambam, the preponderance of the good is questioned only by the ignorant populace... Rambam goes on to argue that the truly bad things that happen to people are not God's fault but, in the majority of cases, their own...
Among the rishonim, Rambam's view is not beyond dispute. Thus, for example, Saadia contends that belief in reward after death is rationally necessary because all good in this world is mingled with bad and the sadness outweighs the joy. Only the prospect of future existence reassures us that "after all in the end justice is done."...
At first blush, it would appear that Rambam's cheerfulness and Saadia's somber diagnosis stand in straightforward contradiction and that only a stubborn, harmonizing piety would undertake to bridge the gap between them... Nonetheless, the dejection about the state of this world that we encounter in Saadia is not altogether incomprehensible from Rambam's viewpoint.
The crucial point is that Saadia does not claim that man looks upon creation and beholds, contrary to the seeming implication of God's judgment on the sixth day of creation, that it is more bad than good. The world that Saadia investigates and finds wanting is this life, when viewed in isolation from the reality of the world to come. Real life is the whole, comprising both this world and the other one... [T]he standpoint of eternity suffuses our experience of this world... In short, the value of this world is contingent on the meaning inculcated by our vision of the world to come.