That's a good point, and for that matter, what's the cause of the major "stress" that balls suffer if they have no feelings. Being moved from one home to another or loud, raucous noises shouldn't affect them at all. Isn't "stress" an emotion caused by other emotions compiling & not an instinct?
Stress isn't an emotion. It's actually a chemical reaction in the brain to something negative that may cause harm. Stress is uncomfortable and animals that go through stress automatically seek to relieve that stress by running away, biting, hiding, etc. It's a release of chemicals and nothing more complex than that. Humans make it more complex because we have the ability to think. A human stuck in traffic experiences the same type of stress as a zebra being chased by a lion - and there is no reason for it, it's just something humans do unnecessarily. We put emotions into it - stress itself isn't emotion and certainly wouldn't be caused by other emotions compiling. In the human world, you might stress because you're job is on the line and things are tense with your significant other but these things are stressful to you because your brain has told your body that there is some unfathomable reason to release that chemical, not because you're emotional about it.

It's very difficult to explain. Humans are very dumb while being very intelligent at the same time. As I said before - I think we feel like everything else does, but we try to rationalize and understand what we feel, so we've lost the understanding that most of it is just instinct, chemicals and functions meant to keep us alive and thriving. Nothing more. People are animals, that's all. Therefore, animals are animals - that's all. Because we think complexly and nothing else does we try so hard to find this unique gift in other creatures and the cold fact is that we're alone in our understanding and intelligence - and we simply cannot seem to accept that.