In addition to the other excellent coment, I'd like to point out that "emotions", along with other subjective experiences, even including consciousness itself, are extremely difficult to measure.

We can really only measure an organism's responses to stimuli (including, increasingly, physiological/neural responses). We can't really say for sure "this dog is experiencing loneliness because she was left home for two days", so much as we can say "this dog is whining, laying about, and staring off into space more and more the longer she's left alone". We can't even "prove" other humans experience consciousness the same way we do, as we don't actually kniw what causes it, but they act similarly, so the simplest explanation is that they do.

It's not hard to imagine an organism that experiences the same emotions as us, but that reacts to those feelings completely differently. In that case, how would we know what they're feeling? Similarly, how do we know trees don't have complex internal lives, but because they can't move, we've no idea?

Ultimately, we assume more complex nervous systems mean more complex consciousness and emotions, but the details aren't well established.