In the past most stress was caused by physical threats. While we still encounter physical threats (watch that car!) most of them we can predict and successfully avoid.
Modern day stress is caused by emotional, mental and social pressures. These pressures trigger the same response as physical stress, but they tend to last much-much longer and that's why it's such a problem.
Stress has moved from the domain of the subconscious into the domain of the conscious, so understanding the psychology of stress becomes more important. As the effects of stress are prolonged, the benefits of the 'fight-or-flight' response mechanism go away, and all that is left are the mental and emotional traps we built for ourselves to keep going and suppressing our doubts and desires.
Unfortunately, that doesn't work. The conscious mind battles with the subconscious mind, and the psychological stress of that battle causes damage on all levels of our being.