Negative thoughts stick around because we believe them, not because we want them or choose them.
Stress is the negative whirlwind of emotions that gets imposed on top of our stimulation and engagement.
Remember that stress doesn't come from what's going on in your life. It comes from your thoughts about what's going on in your life.
The number of stressors has multiplied exponentially: traffic, money, success, work/life balance, the economy, the environment, parenting, family conflict, relationships, disease. As the nature of human life has become far more complicated, our ancient stress response hasn't been able to keep up.
Stress is not the spice of life any more than arsenic is. And without it, you won't feel bored.