This is not a very popular answer but happiness is not a choice of free will. As much as I dislike the Watchtower Society and think that it can destroy people's lives, it cannot, by itself, make one happy or unhappy. Though a very complex emotion, happiness is ultimately a brain chemical reaction largely determined by circumstances beyond our control such as genetics and our early developmental environments.
I think many naturally unhappy people self-select themselves for membership in the Watchtower cult because they are struggling to find radical solutions for their unhappiness. Unfortunately, these self-selecters often drag along others (family members and friends) into the cult who may not be as predisposed to feeling bad and these people can be made miserable by the high control and social pressure exerted by the Society. Extended periods of high stress may create body chemicals which can perminently alter our ability to feel joy but I think this is less a factor than plain old genetics.
The Watchtower cult probably has a larger percentage of mentally ill people than the same number of any given random population for these and many other complex reasons, but certainly, there can be perfectly happy healthy JWs who just happen to have personalities which are well suited to this lifestyle. I, on the other hand, am much happier after having left the Watchtower cult because I am naturally curious, adventurous, and an intelectual/cultural (as opposed to a physical jumping out of airplanes) risk taker. I am also somewhat antisocial (an introvert?), I do not trust authority figures, and I question eveything I am told by having to prove it for myself. These personality traits are deadly if one is to remain an effective and satisfied JW so I was guilty, stressed, and miserable which promted me to seek a way out of the "Christian Brotherhood" which for me was a living Hell.
If we are given some small choice in finding happiness, considering the brain chemistry we are stuck with, it would be in discovering and then adopting a lifestyle which matches our complex and unique personalities. There is no room in the Watchtower Society for unique individuals or diverse lifestyles and beliefs. If you are unable to conform yourself to the basic JW stereotype you will be unhappy. If you are already naturally unhappy and cannot fool yourself into believing that being a JW will make you happy then you will become clinically depressed, and some will just plain go crazy in that pressure cooker called the Watchtower Society. Most normal humans will not be happy as brainwashed dubs but there are a few folks who like it just fine. If it fits them and they don't mind living a lie then there is no way to get them out.
If someone is naturally depressed leaving the Watchtower will probably not cure them and make them happy, just as becoming a JW couldn't make them happy in the first place. Being rich or poor or having all your dreams come true probably won't make everyone happy or unhappy either. To some extent, our ability to be happy is predetermined by factors beyond our control whether we are religious or not. Christianity by itself will not make you happy nor will Atheism.