As far as we can tell, from a purely scientific viewpoint, human life has absolutely no meaning. Humans are the outcome of blind evolutionary processes that operate without goal or purpose. Our actions are not part of some divine cosmic plan, and if planet Earth were to blow up tomorrow morning, the universe would probably keep going about its business as usual. As far as we can tell at this point, human subjectivity would not be missed. Hence any meaning that people ascribe to their lives is just a delusion. The other-worldly meanings medieval people found in their lives were no more deluded than the modern humanist, nationalist and capitalist meanings modern people find. The scientist who says her life is meaningful because she increases the store of human knowledge, the soldier who declares that his life is meaningful because he fights to defend his homeland, and the entrepreneur who finds meaning in building a new company are no less delusional than their medieval counterparts who found meaning in reading scriptures, going on a crusade or building a new cathedral.
So perhaps happiness is synchronising one’s personal delusions of meaning with the prevailing collective delusions. As long as my personal narrative is in line with the narratives of the people around me, I can convince myself that my life is meaningful, and find happiness in that conviction.
This is quite a depressing conclusion. Does happiness really depend on self-delusion?

No man is happy without a delusion of some kind. Delusions are as necessary to our happiness as realities.

Happiness depends upon ourselves

The brain is very good at self-delusion.

Self-deception is a pessimistic definition of optimism.

Human beings have an almost unlimited capacity for self-delusion. We can justify any amount of sadness if it fits our own particular standard of reality.

happiness does not really depend on objective conditions of either wealth, health or even community. Rather, it depends on the correlation between objective conditions and subjective expectations.

Happiness does not rely on what you have or who you are. It relies on what you think.

So perhaps happiness is synchronizing one's personal delusions of meaning with the prevailing collective delusions. As long as my personal narrative is in line with the narratives of the people around me, I can convince myself that my life is meaningful, and find happiness in that conviction.

Though it is folly to suppose that happiness is a matter of volition, and that we can make ourselves content and cheerful whenever we choose — a theory that many poor hypochondriacs are taunted with till they are nigh driven mad — yet, on the other hand, no sane mind is ever left without the power of self-discipline and self-control in a measure, which measure increases in proportion as it is exercised.

Happiness does not depend on what you have or who you are, it solely relies on what you think.

Remember, happiness doesn't depend upon who you are or what you have, it depends solely upon what you think.

I think anyone who is creative or self-aware in any way, there’s like a humility to it, or I should say a humiliation to it. But there’s also a self-delusion — the provisional ego, as my uncle would call it. The self-delusion is the thing that makes you go, oh you know what, all the music that I’ve ever loved in the world, I want to be a part of that — hey, listen to what I have to say, it’s really important, it’s going to matter.” You can’t apologize your way into people’s hearts ... You have to go full force.

Happiness does not depend on any external conditions, it is governed by our mental attitude