An application note from Rubycon about what causes electrolytic capacitors to deteriorate. PDF link is Here.
Performance deterioration of aluminum electrolytic capacitor is accelerated by repeated charge and discharge. The speed of deterioration is determined by operating conditions. For example, the higher the voltage and smaller the discharge resistance, the tougher it is on the capacitor. Also the deterioration is accelerated when charge and discharge takes place very quickly, or in high ambient temperatures.