Although a somewhat Eeyore-like and gloomy individual in many ways; I (thank Heavens) have been prone to outbreaks of unreasonable happiness through my life - those times various called, Peak Experiences (Maslow), Epiphanies (James Joyce), Joy or Sehnsucht (CS Lewis) and many other things - those feelings much loved by the Romantics such as Wordsworth and the New England Transcendentalists such as Emerson and Thoreau.
The mainstream secular modern view is that such moments have no deep significance - being caused by physiological change, or drugs, or a brainstorm... or something like that. The attitude is to enjoy them while they last, and then forget about them - because there is no special significance, nothing to be learned from them.
Ralph Waldo Emerson regarded these moments as significant - but cautioned against regarding them as evidence for optimism about future life. For Emerson they were not harbingers of anything, but to be valued in and of themselves - they were on the one hand the most important things in life, on the other hand utterly self-sufficient and free of general implications.
In practice, however, this remained little more than a bare literary assertion: meanwhile such moments came and went, the complications of life continued and then ended. Was Mankind any further forward after Emerson's explanations and Thoreau's experiments - were Emerson and Thoreau themselves any further forward? Seemingly not.
Yet, as the secular, this-worldly, spirituality which Emerson pioneered began to gather strength, there emerged a view that such moments of unreasonable happiness were harbingers - of a possible 'evolutionary' future for Man: a future of Men developing a higher and much happier consciousness. Such was the theme of Colin Wilson, and his books from The Outsider onwards collected and analyzed many examples of this.
Indeed, some kind of fundamental change to the human mind, society, or some combination of the two (whether or not this change was called evolution) was necessary if unreasonable happiness was to be anything more than a glorious interlude.
Yet, even this has been subverted by Transhumanism - which purports to engineer and make permanent such moments in a scientific project dedicated to transforming human physiology using drugs, genetics and anything else necessary.
Even supposing this were possible; the implicit conviction is that unreasonable happiness is a delusional epiphenomenon - the hope is to make it a permanent delusion (and without the shadow of death to cloud perfect bliss, because death would be abolished too).
A project genetically and pharmacologically to engineer a state of permanent happy delusion in Mankind is itself perhaps the bleakest and most despairing philosophy of life which humans have yet devised.
What of Christian views of such moments? There are many and diverse - from the pessimistic idea that moments of happiness are most likely to be demonic deceptions; to the optimistic and positive visionary theology of Thomas Traherne.
But my particular interest is the idea from CS Lewis, and less explicitly from Tolkien, that such moments of happiness are evidence for Christianity - the 'argument from desire'.
That such moments of happiness are actually a desire-for - and foretaste-of - something not-of-this-world (since nothing imaginable in this-world could be a fulfilment of this desire); and the fact that we have such other-worldly yearnings is evidence that that desired world is real (otherwise we would not have such feelings).
In a nutshell, we are unreasonably happy because of hope for Heaven - and that hope was implanted in us by God.
I personally find this a compelling argument (while being aware that others do not) - but as expressed in Lewis and Tolkien, it is somewhat incomplete. The reason it is incomplete is that this feeling is not only forward-looking and hopeful but (and Lewis says this very clearly) is also very powerfully nostalgic and backward-looking. Our state of unreasonable happiness is a yearning for what was, as well as what may-be.
Tolkien expresses this as a yearning for Eden - a real state of Paradise in which Man dwelt and from which he now is exiled. Tolkien seemingly regards this as a kind of inherited race-memory (indeed, the concept of hereditary memory is central for Tolkien's world view).
But I find the complete explanation for the unreasonable happiness of Joy, Sehnsucht, Peak Experiences in the doctrine of a pre-mortal spiritual and Heavenly existence.
This was also the implicit explanation of Wordsworth in his famous phrase: Our birth is but a sleep and a forgetting: the soul that rises with us, our life's star, hath had elsewhere its setting, and cometh from afar: Not in entire forgetfulness, and not in utter nakedness, but trailing clouds of glory do we come from God, who is our home.
This I find to be a full and satisfying explanation for the phenomenology and function of these significant-seeming happy experiences: that the special quality of both backward nostalgic yearning and forward hopeful yearning, combine to locate my mortal life in-the-middle - between partial memories of a pre-mortal spirit-existence in Heaven before my birth and optimistic anticipations of a resurrected Heavenly life after my death.