An infant's anger is usually triggered by

frustration. When an infant is angry, crying or fussy, there are many reasons that can be fueling it, but in the end, the anger is caused by frustration. An infant surely doesn't know WHY it is uncomfortable or how it can be remedied, only that he or she IS uncomfortable. Therefore, the only relief comes from the one communication it is capable of, crying. As the infant becomes more aware of the reason for his or her discomfort, the child might also show anger because it cannot communicate the message.