I think people's definition and understanding of "emotions" varies a lot, hence why they may be hesitant to think animals have emotions. They may be thinking of something complex or nuanced. But some emotions are basic and primal, and clearly exhibited by animals. For example, fear. Fear is a basic emotion that lots of living creatures have. It stems from the instinct for self-preservation, but it's more than just that. Plants have self-preservation reactions to danger, like how the smell of cut grass is actually a chemical that the cut grass sends out to alert other grass in the area that something is eating/cutting it. But that wouldn't really be classified as fear. A chicken that is undergoing a traumatic experience, or is remembering a past traumatic experience, and responds with screams, widened eyes, running away, cowering, increased pulse and heart rate, etc. looks a whole lot like a creature experiencing the emotion of fear. Chickens have the hardware necessary to experience the emotion of fear (they have a brain that has an amygdala). And they show behaviors consistent with the emotion of fear. So how can we say that they aren't, in fact, experiencing fear? And thus, emotions? Sure, maybe their emotions aren't as varied, complex, or deep, as ours, or as those of a dog, but they definitely show evidence of experiencing at least some emotions.