What causes a guy to change into this?

i was married to a guy 10 yrs i thought he was mature, stable, he didn't hang out with the guys, he just came home. but then he began acting differently, all of a sudden he began to hang out with the guys, he began doing drugs. turned out he had a girlfriend and she did all these things. u didn't loose anything but a hard time and a life with a man who was probably always like this , u just didn't see it. they claim u become just like the person your sleeping with, my ex did. he began to act like an immature boy, and the other woman was allot younger. just consider yourself lucky u did not marry him.