You are not exercising long or hard enough to get an endorphin rush.  Exercise causes endorphins, which create a sense of well-being, to be released from the brain.  Endorphin release, called the “Runner’s High”, occurs during long continuous workouts when the level of intensity is between moderate and high and breathing is difficult.  (Endorphin release also corresponds to the time when muscle glycogen is depleted.)  To improve your stamina for long aerobic workouts, use a training schedule, build muscle with weight-training, eat appropriately before and after working out, and give it time.