The etymological root of the English 'happiness' is the Middle English 'hap', which means luck, fortune or chance[…] [A] happy mode of being is one in which I am able to receive the fact of the world – its happening – in the right way: the happy are those who live this fact as something lucky or fortuitous, as something that could have been otherwise, but (happily) was not.

'Hap' can also mean 'absence of design or intent in relation to a particular event': what haps does so for no reason; it is literally graceful. The happiness in question is the happiness of living the fact that existence is unnecessary or gratuitous: not (empirical) happiness at the occurrence of this or that thing, but (transcendental) happiness at their happening.

[…]

My existing, my absolutely particular response to the groundlessness of existence, cannot be exchanged. It is non-relational and therefore in a sense it is entirely private, unique to me alone. Yet so is yours, and that of any other living being. What is common, in other words, is our singularity in the face of the fact of the world; we are absolutely substitutable right where and when we are most unique.

This is another way of understanding this idea of happiness: it is a happiness that singularises the self, but that does not exist except in common; it is the happiness, then, of the sharing of singularity, the happiness of our common exposure to the grace of the world. […] Living in proper response to it would invovle the common appropriation of belonging itself: not any particular condition of belonging, but the fact of our being in common.

From Matthew Abbott's The Figure of This World