Well I know for me, Being in Athletics from 7th-12th kept me in great shape and it was all planned out for you, I basically got to work out during school hours and had free access to gym, it was just part of the routine.
After you leave, you have make time to work out, you have find a gym and pay to workout. You don't have coaches or teammates pushing you to go harder and you don't have those structured workouts so you have put more effort forth to stay in shape..
I still stayed pretty active during college and when I first started working. But once you start working that 40 a week, have a kid, females you deal with, starting going out and hanging with friends, drinking, smoking, grabbing late night fast food it gets harder to take time to workout. Especially when you like 19-25
I'm good myself cause I still workout 3 times a week now and walk a mile everyday at work and work at a job that promotes fitness and health.
Also I don't really believe that relationship/married stuff, I workout more now that I'm married than I did when I was single. If you you got a fine wifey or girlfriend that works out, trust me you will be motivated to stay in shape and vice versa
But its not hard to see how people can fall in to being complacent and then get