The answers to such questions vary extremely widely; from those who say that being a Christian is strongly associated with greater happiness, more hope, love and courage; through those who say it has no necessary effect either way; to those who emphasise that to be a Christian has often been to invite isolation, persecution, suffering, and a miserable existence.
There are several reasons why the answers vary - the first is that there may be a big difference between what happens-to a person and how they feel inside. There is no necessary correlation, positive or negative, between the external conditions of a person's life and their inner state. Some of the happiest of people have been among the poorest and weakest and most despised; and many of the most prosperous, comfortable, powerful and high status people are utterly miserable.
Another reason is that there is a kind of here-and-now surface pleasure or misery; but there is also a deep, tidal happiness or despair. To put it differently, life can be seen as bounded by birth and death; or it may be known as extending across infinite time - and the infinite perspective recontextualises the ripplings of immediate positive and negative emotions.
But there is a deep reason for the variation in effect that becoming a Christian, or being a Christian, has on people - and this relates to the variation in 'the self' they inhabit.
Most people in the modern Western world identify with a false self in themselves (a superficial, fake, socially-inculcated, often labile 'personality') - and this also applies to most Christians. But some Christians live (some or much of the time) from their real self. The real self is the divine within us - God-within-us, by which we are children of God. The false self is false, but the real self is true.
Probably nobody lives all the time from their real self; and this state is usually something intermittent and partial. But when a Christian is living from their real (and divine) self; he will be happy - even when he is also suffering.
Because to live from the real self in knowledge of the truth of Jesus is to be happy.
I think many Christians miss this fact and necessity; because they neglect the extent to which the modern world alienates us from our real self - that is what such phenomena as the mass media, bureaucracy, totalitarian monitoring and control are all about. And an alienated Christian - a Christian living from a false self - is probably unhappy even though he is a Christian.
In conclusion, Christianity IS therapeutic, and DOES make a person happier WHEN that person is living from his REAL self - but not necessarily when they are not.