Greetings,
that is a tricky question as what is to be defined by "happiness" is fuzzy and also there is the fact that people are different. Some are happy for no apparent reason. Most persons are only happy SITUATIONALLY or in other words their happiness is a response to the environment while those that are capable of being intrinsically happy are probably in the minority.
A religion (doctrine and culture) that focuses on the goal of becoming PERFECT while simultaneously reminding its members of their imperfection is not conducive to promoting a state of individual happiness (or even positive self-esteem) and thus most Jehovah's Witnesses as individuals are not intrinsically happy.
Jehovah's Witnesses are only truly happy and joyful when they are with each other.
This is because the religion (docrtine and culture) is very poor at helping one develop their own inward sense and generation of "happiness". Instead, Jehovah's Witnesses learn that their happiness is dependent upon FUTURE contingencies.
Thus JWs never learn well to live in the moment and appreciate the happiness in this life. How can they (we) when they (we) are taught that this world either has nothing to offer or its good and happy things are only an illusion or a product of wickedness?
The result is that the Jehovah's Witness who is isolated feels all of the anxieties of this world and finds it very difficult to generate or have any happiness in the present. Having nothing in the present moment to be happy about, and only holding the hope of future happiness, only results in a very fractional amount of happiness. It is basically analogous to having something real that one is happy about and hoping for that thing to come to you.. The first position can bring a lot of present happiness while the latter "hope" only brings a fraction of that same happiness, which is really very faint and fragile.
When a Witness gets together with another Witness, there is a shared reinforcement of the future-forward belief system and thus the "faint happiness." The more Witnesses the better as this both compounds the reinforcement and also coincides with a reduction of exposure to the "outside world."
Thus you can see that is why Jehovah's Witnesses feel a measure of happiness at meetings and gatherings and especially more so at assemblies and district conventions.
The further implication of the above is that those that are and truly do feel happy are those that "draw toward the center" of the group and religion and are fully immersed. Those that lay organizationally speaking at the outer rings, edges and fringes are progressively less happy.
Those that leave the Organization are the unhappiest of all UNTIL they rebuild their belief-structure and shift their "Future-forward" outlook on life to one of living in the present and learn that there is much happiness to be had in this world and in this imperfect state.
-Eduardo