Stress is the body's response to any real or perceived threat, demand or danger.
The heart needs to pump more, work more during this stage. As humans have evolved, this fight or flight reaction has become a reflex phoenomenon at any real or perceived threat or demand, ranging from any major life event to day to day minor stresses.
Stresses can be positive- for example, birth of a child, promotion at work, victory in a match or it can be nagtive like death of loved ones, failure at work, financial loss etc
During stress, the body releases some stress hormones like cortisol in the body.
But when the stress is chronic or continues for long time and repeatedly, the stress hormone is present in the body for long time. Now the released hormone causes increase in blood sugar level, high blood pressure.
Because of stress, the cells are carrying for energy, body sends signal to brain that you are hungry and need to eat resulting in high calorie food intake and weight gain- which again is a risk factor for heart disease.