Consider pleasures and pains. Most lives contain both, to varying degrees, but there is an unfortunate asymmetry between these that seems to apply to even the best of lives. The upshot of this is that there is much more pain than pleasure. For example, while the most intense pleasures, such as sexual or gustatory ones, are short-lived, the worst pains have the capacity to be much more enduring. Indeed, pleasures in general tend to be shorter-lived than pains. Chronic pain is common, whereas there is no such thing as chronic pleasure.
Moreover, the worst pains seem to be worse than the best pleasures are good. Anybody who doubts this should consider what choice they would make if they were offered the option of securing an hour of the most sublime pleasures possible in exchange for suffering an hour of the worst pain possible.
Consider how an injury can be incurred in a split second and the effects felt for life. While it is true that we can also avoid an injury in an instant, we do not gain benefits that are comparable in their magnitude and longevity in a mere moment.
Second, even when desires are fulfilled, this usually occurs only after the exercise of effort. This means that there is a period of time in which the desire is not yet fulfilled. Finally, when desires are eventually fulfilled, the satisfaction is typically only transitory. Satisfied desires give way to new desires. (For example, one is hungry, eats to satiety, but then becomes hungry again.) Thus a relatively small proportion of life is spent satisfied.