While WE certainly think running is the best exercise, it is important to first start out by saying that the best exercise for you is whatever gets you moving, keeps you active, and that you enjoy. But it would make sense that a physician would tell you that running is 'the best' because of the myriad of health benefits that it offers, and the great flexibility and variety offered in the sport of running. Basically, you can count on running to help you out both physically and mentally, which can then lead to improved emotional and spiritual health as well. This is why doctors encourage so much!
Physically, running is the ultimate calorie torcher. It blasts calories, and helps you maintain a healthy weight (if that is your goal) or lose weight in an efficient manner, so long as you are eating healthy, balanced meals and not surpassing total calories consumed versus calories burned. But it also incorporates the major muscle groups, and helps build and tone lean muscle mass. It is also a fabulous workout to keep bones strong and healthy, as weight bearing exercises help improve bone health and stop and reverse early onset bone deterioration or bone loss. The physical health benefits that running offers are great, but we also know that running is not an easy sport. It can be very hard, and an intense running schedule can be too much for a lot of people. Fortunately, even just moderate levels of running for 20 minutes a day, three days a week can give you the amazing cardiovascular benefits that running offers - aka improved lung health and capacity, a stronger heart, and more efficient pumping of blood and oxygen throughout the body.
Mentally, running offers a lot of benefits as well though. Any sport that increases heart rate, gets the blood flowing, and the endorphins pumping can help calm anxiety, improve depression, and provide a healthy outlet for stress. Running has also been linked to an improved body and self image, a heightened level of self confidence, and more appreciation of self and surroundings, which leads to feelings of gratefulness (which is a huge component in improving mental health). Running increases serotonin, and can help regulate brain chemicals and hormones that have an impact on mental health.