Pain is not suffering
Questions of theodicy (why bad things happen to good people) are often predicated on an inadequate understanding of the distinction between pain and suffering. The two are different and distinct. The concept of suffering has been extensively analyzed in the medical and bio-ethical literature. Much of this work was done by Cassel.
in brief, the relevant point is that there is a distinction between pain and suffering. Life often brings with it pain, but suffering is existential. Pain is value neutral and can even be a positive experience. Suffering, on the other hand, requires an interpretation of pain. Thus, for example, childbirth is pain but nor suffering, for it is a happy, meaningful occasion, whereas chronic back pain is suffering because it is purposeless and is so perceived. Consequently, as we often observe, even minor pain can lead to disproportional suffering, when it is interpreted as a meaningless and triggers hopelessness, helplessness, and loss of worth. On the other hand, suffering can be turned into… just a pain, if it can interpreted as meaningful and beneficial. This teaches us that one’s perceptions and interpretations of pain can make it a tolerable means to some meaningful benefit, or can turn it into exquisite suffering.
If so, animals cannot suffer for they cannot judge or interpret pain. Animal rights may differ but we are talking philosophically. Sure, animals feel pain. People under anesthesia also often feel pain; their heart rate and blood pressure can be demonstrated to go up, especially when anesthesia is light, but they do not suffer because their cerebrums are turned off. The question why animals suffer pain, however, is of a different magnitude of impact than the one of why they undergo suffering.