Why run?

Running as an exercise can strengthen the limbs, develop the lungs, exercise the will and promote the circulation of the blood. Running is well adapted to both young and middle-aged persons. Sedentary persons may find great benefit in it after the day's work is ended. Girls can run as well as boys, and while they cannot go so fast, they can run much more gracefully.

--Scientific American (1883)

There are as many reasons to run as there are runners, writes Bill Rodgers in the first chapter of The Complete Idiot's Guide to Jogging and Running.

Being an idiot is definitely not one of them, though running bashers would think and say you are for wanting to punish your body.

Physical reasons top the list. People run to lose weight, ward off heart disease, or simply feel better.

The Complete Idiot's Guide to Jogging and Running lists what it calls a baker's dozen of the some of the most popular reasons to run. I have taken the liberty of adding some notes to selected items.

1. Running is a great way to lose weight.
2. Regular aerobic exercise, such as running, decreases the risk of heart disease.
3. Running is one of the most effective ways to improve cardiovascular fitness.
4. Running is a great way to quit smoking. (Worked for me. I was a pack-a-day smoker when I started to run.) 
5. Regular aerobic exercise reduces the risk of some cancers.
6. When you become fitter by running, you're more likely to improve other aspects of your lifestyle such as your diet.
7. Running is the most convenient of sports; you can do it almost anytime, anywhere. (I run whenever and wherever I can so long as my trusty running shoes and running apparel are there.) 
8. People who have a high level of physical fitness usually have a positive self-image.
9. Running is cheap; all you need are a good pair of running shoes and a few pieces of running apparel. (Another one that works for me. I don't have that much money to spend.)
10. Fitter people are more productive on the job.
11. Running with others is a great way to build friendships.
12. Being fit increases mental sharpness; regular exercisers are able to concentrate longer and are better problem solvers.
13. Running is one of the best stress relievers around. 

Some of these reasons can also apply to other forms of exercise.

Cycling gives pretty much the same benefits, but in running, you don't have to worry about how to get home with a flat tire.

Tennis and badminton? Yes, but then in running, you won't have to regularly buy new balls or shuttlecocks or pay for time in an air conditioned court every time you play.

About.com has a page which discusses the health and social benefits of running.

The Trinity College Dublin website mentions psychological benefits and backs these up with some research data.

Ezine @rticles also has an article "Why Run?" by sports enthusiast Nathan Hangen.

Hangen's closing paragraph in this article perhaps best expresses the attitude one should have about starting to run.

Running has many benefits and there are many more than I've listed here. It really is a life changer in more ways than one. Don't let the fear of getting started stop you from seeing these benefits in your life. The key is like Nike says, "Just Do It."