Because there is no God and life is suffering?
"A quick test of the assertion that enjoyment outweighs pain in this world, or that they are at any rate balanced, would be to compare the feelings of an animal engaged in eating another with those of the animal being eaten."
Schopenhauer
1. The majority of animals will be dead for the majority of time while being eaten and digested (Or is not a full stomach a pleasure?). Think of scavengers or of many small creatures eating a larger, the pleasure being multiplied by the addition of eating creature. "Eating" lasts longer than being eaten, even if it is a lesser pleasure.
2. If the domain is suffering and pleasure in "the world" let us, more fairly, take the entire animal kingdom. One herbivore could have hundreds of delicious meals on painless plants while being eaten only once. It might be replied that there are many sufferings that an animal can experience, such as cold and fear. Well, they can also experience cozy warmth and feelings of security. I'm sure we can imagine what it would be like to experience the simple pain and pleasures without all these human concerns floating around in our head. Most people desire as such, as they understand the fear of misfortune and pain in us is by far the greatest part of our suffering. I don't know about you, but I never really minded the cold as long as I felt alive during. We watch horror movies to experience the exhilaration of fear without fearing the consequences. I would say a rabbit fears the predator, not the consequences of being caught. It knows only
run. That fear is pleasure
and pain, like our fear of scary films.
3. As interesting as looking at pleasure and pain broadly is, a more specific focus might be less misleading. It might be tempting to take S's quote as representative of both human and wider animal lives. This would be a gross error. A humans life is an unnatural state in which we cater to our own pleasure and create measures for pain avoidance (pain killers being a literal example of this). We will experience thousands of meals created with pleasure in mind before our deaths. Pointing to the suffering of those in less fortunate areas of the world is not sufficient to disprove my general point--it being that life doesn't have to be and will not be defined by pain as long as basic physical means are met. Once these needs are met, which, I expect, is true for every member of this discussion, only the troubled human mind is pain-creating. This is the problem of the human as believer and it is what haunts the members of this thread. Granted, it isn't difficult to prove many human lives are defined by pain caused by this issue. What you cannot prove is that this is necessarily the case as long as a mind is rational, honest, sympathetic/empathetic and intelligent. I do think I am capable of defending my optimism as just as rational and aware, no less delusional, than your pessimism. (We would, of course, call both our points of view "realistic" amongst ourselves)
*edit* I would also include freedom as a basic human need. Vast quantities of human unhappiness has been caused by social norms that restrict freedom beyond the useful.