The best runners on my local team are inherently a little off balance. One guy obsesses over mileage and is very OCD with other things in his life to the extent that his social relations with others are strained...a real odd ball. Another girl is someone who is basically addicted to cardio. She needs it to feel normal, a lot of it. She's a very fine runner. I have personally been on and off anti-depressants for years due to chronic worry and depression. Running and training helps me to not worry and enjoy my life more...kind of like a natural Prozac...if I don't run I have issues sometimes.
In these ways, depression, addiction, alcoholism, OCD are all mental disorders people have. Running tends to attract these types due to the training, but also due to the balancing effect on neurotransmitters in the brain. When these types of people run (like me) they feel "normal" and happier.