Because emotions are produced by hormones and neurotransmitters that circulate in your body. When there are too powerful emotions, the organism can get overwhelmed by this bioactivity. Stress, in particular, increases cortisol and norepinephrin levels in your body, which if too high for too long period can put serious stress on your system and also deplete your immune system in the longer term. That is why learning to manage stress is important.
You have to learn to manage your response to stress. Because, in the end, it is your psyche that triggers the production of emotions in your organism. You can control it.