One of my worst fears is to be reincarnated as an insect or some other small prey animal. You are guaranteed a horrible, painful death. You are most likely to get destroyed by some other animal, if you don't smash on a windshield or become some toddler's torture toy.
And animals are not kind to each other. If you can avoid getting your head ripped off after mating, or torn apart in a ant vs. termite war, you're guaranteed to get eaten. You'll become old and slow, and eventually you won't be able to avoid that flicking tongue.
And it's not like there'll be a crunch and instant death either. You are alive when they start to eat you. You won't die right away, or shut down. The frog or snake's going to swallow you whole, while you feebly struggle in its cavernous mouth. You'll find a new definition of pain as you are slowly digested.
The sad part is that if you are reincarnated, you are almost guaranteed to become some other animal's food. (Assuming that the border limit is at least multicellular animals) The odds just work that way. There are a lot more insects and fish in the world than humans.
I hope animals don't feel pain. At least not in the same way we do. They go through so much agony -- constantly searching for food and starving in-between, craving to mate. And they die constantly.