Everyone experiences stress through their daily live's; it is just our nature. Learning how to respond to individual stressors is what we need to do. Stress can come from everywhere life events, work, financial, relationships, etc. Each stress taxes the body causing fatigue, sometimes even depression. Having too much stress when exercising can actually negatively affect your training results, as it slows down your recovery time thus increasing the chances of overtraining syndrome and ultimately injury. This article by Shirley Archer, JD, MA

http://www.ideafit.com/fitness-library/how-well-you-manage-stress-may-affect-training-results