There are as many reasons as people training out there. Few of those are:
Participation in a race or other athletic events
Curiosity (try different types of training styles)
Health issues (obesity, etc.)
The feeling they(military personnel, police officers and firefighters, etc.)
The feeling of belonging to a particular group (such as the Crossfit community)
this is a very vague question but here are some… fear, pride, ego, endorphins, necessity…there are too many to list them all.
I personally train because I love the feel of heavy iron on my back…guess thats ego huh lol. I also train out of necessity because if I don’t train my back will prevent me from being able to enjoy my 2 year old daughter.
In addtion to the excellent responses, I would only add two things. One is that in order for whatever modality to become part of an individual’s lifestyle, it has to be an activity that’s looked forward as enjoyable, not as a drudge.
Second is that the type(s), intensity, duration and frequency don’t lead to pain or injury.
Take care, Daniel