It was an ancient insight (going back to the earliest recorded philosophy) that Men as we are in this earthly mortal life; cannot be truly happy.
Because; if Men (as we now are) were placed into 'Paradise' - then it would not be Paradise.
We would not be fully happy there - and might even be more miserable.
Furthermore; even if it had been Paradise before we arrived - we (as we now are) would soon wreck the place, to at least some degree -- in which case it would no longer be Paradise.
Therefore; we our-selves are to blame for the ultimate unsatisfactoriness of mortal life on earth.
Therefore, we have need for transformation before we can be truly happy; we our-selves must be first transformed before Paradise could be Paradise.
Interestingly, this is recognized by the most advanced form of materialism so-far: transhumanism. This recognizes that Men need to transcend their current nature if they are to be fully happy, and free from suffering.
But the transhumanist sees this as a material problem; and envisages Men as being transformed by physical means: drugs, surgery, implants, genetic engineering etc. But physical interventions can only operate within the constraint of this mortal world, which is entropic by nature; such that the desired order is always being-corrupted by chaos and any desirable state of being is temporary.
Thus, even by its own lights - transhumanism can only at best yield amelioration of our condition - not Paradise.
If Men are to be happy in Paradise it requires a spiritual - as well as material - transformation; indeed it requires that we understand the material to be part-of the spiritual, with the spiritual as primary.
The Christian transformation that enables us to be wholly happy is termed resurrection; which can be understood as primarily a spiritual transformation to 'fit us for Heaven'/ Paradise - as well as the necessary physical transformation for such a life; such that we will have bodies, and yet also be immortal.
Since Christians are called upon to have hope, and to follow Jesus to salvation; it seems to me that we need to be able to imagine what Heaven is like - sufficiently in order to desire it.
Can we imagine the transformation of resurrection and a fully happy life in Heaven?
Some of us can. Those who recognize that some-times, at our best, this mortal life on earth is indeed Paradisal; thus wholly happy - albeit briefly.
And we may also be able to recognize that these moments are also those times when we are most our-selves...
We may therefore be able to imagine, hence understand, that - in principle - we could be transformed such as to remain our-selves at our best; and become fit for Paradise.
We may also be able to infer from such moments (and their basis in love) the nature of paradise.
In sum - we may be able to know both that we require transformation to be truly happy; the nature of transformation needed; and that this kind of transformation is exactly what was offered by Jesus Christ.
(After which we only need to determine whether Jesus's offer was valid.)
Note added. This post approaches the question from the bottom line moral assumption of mainstream modern secular materialist leftism - which is (roughly) that that is good which is most conducive to happiness - especially the elimination of suffering (the conceptualization of what-ought-to-be-happy vacillates, incoherently, between the individual - especially oppressed, victimized - person, and some abstract group entity that is regarded as oppressed/ victimized. But the scope of happiness is assumed to be that of this mortal life; and eternal life is disbelieved or disregarded. Thus all secular ideologies, including those that regard themselves as of The Right (conservative, republican, libertarian, alt-right etc), only differ in terms of suggested means to the same end: i.e. optimizing mortal earthly happiness.
When the assumption is that we live eternally, the main rival to Christianity is that 'Oneness' spirituality which the West has extracted from Eastern religions such as Hinduism and Buddhism, and which is promoted via the New Age, Perennial philosophy, and officially-approved 'mindfulness' exhortations. These locate suffering in consciousness, and consciousness in the-self (plus/minus the-material); and aim at the alleviation of suffering (plus/minus the maximization of happiness) by the annihilation of The Self into immaterial spirit, and assimilation of that which was separate into the oneness of deity.
My point is that there is a sense in which all religions and ideologies can be reduced - and this is a reduction - to the nature of their concern with happiness. And that these differ irreconcilably about the proper scope of happiness, and how it can or should be attained. In other words, there is no possible coherent way of creating a single spirituality/ religion/ ideology from these three fundamentally differing assumptions.
Leftism, Oneness and Christianity cannot be combined coherently - neither as pairs nor as a single unity. All attempts to combine them are actually subordinating one or other; or else alternating between incompatibles.
We must therefore actively and consciously choose what kind of happiness we really want - or else the choice will (passively unconsciously) be imposed upon us.