For mortal human life to have meaning it seems that there must be both permanence and personal relevance for some things in that life.
If everything is washed away at death, then there can be no meaning - everything is just a momentary spark of sensation - a brief sensation, which might well be a delusion.
If all that is left is located in biological memory, then this depends on brains which are fragile and temporary, and memories are fallible and may be false.
So (for mortal life to have meaning) there must be some realm or place or time in which at least some thing are 'stored' permanently (some kind of 'Platonic' realm of true reality, beyond the changes and decays of mortal life).
And this must have memories which are true, real, accurate and valid - which means that there must be a possibility of direct, unmediated transmission of information or knowledge.
(Because any 'normal' material processes - working by means of the usual perceptions and senses and the usual modalities such as light, sound and touch - must be incomplete and distorted, and indeed may be wholly illusory.)
But an accurate and true reality 'somewhere' is not enough - that reality must also be linked to us as individuals, and to our specific mortal lives - or else mortal life is meaningless.
(Some religions, some types of Christianity, are like this: mortal life is rendered pointless and functionless by comparison with the perfection of Heaven - except, perhaps, that evil choices in mortal life are able/ likely to wreck our chances of reaching Heaven. This does provide a kind of meaning for mortal life - albeit entirely negative.)
All this (and more) is implicit when we feel that life has meaning: that at least some of the things that happen in our lives are significant, and recorded in a true and permanent way that also has continued personal relevance to us - so an afterlife is essential as well.
It is clear from the above that some kind of religion - and not just any kind of religion, because some religions are inadequate - is necessary for there to be meaning in life.
Thus meaning in life is not 'given' - nor is it natural - but it requires some kind of revelation.
Lacking which, we will have the background awareness that life is ultimately meaningless, and what we do during it is merely a series of momentary distractions: in a world, nihilism.
The possibilities are therefore nihilism or religion (but only some religions).
