A number of studies have found that moderate to vigorous exercise can benefit people who have clinical depression (follow this link for a Google Scholar search on the topic) and in Australia exercise is commonly recommended in public mental health campaigns and by health professionals. Although research published in the British Medical Journal found that shortcomings in many of the studies make it difficult to draw any firm conclusions about the benefits of exercise (see a summary of the paper
At an anecdotal level, I've talked to many people, in person and via the CoolRunning Australia forum, who find that running helps them to feel better and avoid, or at least temper, the effects of depression. This has been my experience too.
Running hasn't been a cure-all for me: several attempts to dispense with medication have ended with depression reasserting its hold. On the other hand, medication without running provides stability but also casts a sort of dull pall over everything, as if not only the lows but also the vibrancy and colour of life are being suppressed.
For me, running seems to complement other measures in some way, but I'm not convinced that this is wholly due to the effects of exercise. In my twenties and thirties I used to do a lot of cycling and was much fitter than I am now, but I was still prone to panic attacks and episodes of depression. So why does running help ? Here's what I think are my reasons...
Running gets me outside. I find that, on a sunny day in particular, that's often enough to get a definite lift in my mood. Even when it's cold, windy and pissing down I still feel better for getting out into the elements - though perhaps in that case it's the slightly eccentric and ludicrous aspect of it that appeals to me.
I don't think I'd continue running for very long if I was only ever doing it alone. Regardless of the physical and mental benefits that it brings, and even the mild addiction that it can become, I'm sure my inherent laziness would prevail. This is where races come in. John Bingham, in his book The Courage to Start describes races as a public celebration of running. It's always exciting to turn up to an event, whether big or small. Each race provides a clearly defined challenge. You know what you have to do and, for the longer events, you've probably had to work through a structured preparation for weeks or even months previously. Races provide structure and purpose.
You don't have to be a great runner, or even a very good one, to enjoy races. I'm a slow, back-of-the-pack specialist but I find reaching the finish of a race is always a deeply satisfying experience, especially if I feel that I've done my best. There is also the pleasure of being part of an event, of running a particularly nice course, and of seeing and chatting to other runners. Some of my slowest races have been my most enjoyable for these reasons.
Being part of something
There is a community of runners. It's a broad church and includes people from all backgrounds, walks of life and points of view. Some runners are motivated almost solely by competition, be it against others or with their own previous best times. Others, like me, are in it for different reasons. All of us, fast or slow, are runners. We have a shared enthusiasm, a hoard of anecdotes about our successes and disasters, ambitions of races that we'd like to do or results we'd like to achieve, and an anatomists esoteric knowledge of running injuries !
In a nutshell...
Perhaps all of the above can be summed up very simply as: nature, purpose, and friendship.