Why are emotions highly unpredictable?

They aren't, always. Healthy people vary in emotional responses, but these are usually within understandable limits. You can empathize with others' responses, because you may have experienced something similar in your own life. However, some illnesses needing treatment can also cause unpredictable outbursts: bipolar d/o; psychosis; personality d/o; parkinson's disease; post-stroke; als, and dementias.