It is relatively easy to understand why we find each other beautiful - symmetry being associated with health and therefore good genes, etc.

I do not think it is easy to understand why we find another human beautiful as a generality. Symmetry is only one of a quite a few factors and it can be influenced by other factors such as situation.

The more identical the immune system between two people, the less appealing the other person will be perceived. The greater the difference, the more appealing. I do not know if we know to what extent this can be detectable by a human. This type of detection may only be present by the female sex.

So, the positive value increases in proportion to the decrease of symmetry.

We do know why perfect symmetry would be a turnoff, however, if we are speaking in terms of easy to under then I believe a prediction made from this line of thinking would result in an error.

A side note on symmetry deeming mention: Perfect symmetry has a negative effect in respect to the beauty equation.

But why do we find things beautiful when they have no obvious survival or reproductive benefit

Byproduct.

But why do we find things beautiful when they have no obvious survival or reproductive benefit

Why can we find things beautiful that have obvious survival or reproductive benefit, while those are not the reasons they are held as beautiful?

But why do we find things beautiful when they have no obvious survival or reproductive benefit

Why can we find candy bars and soda pop to taste so yummy, while having no obvious survival or reproductive benefit?

(e.g. the stars)? It would seem odd for it to be a neurological side-effect of finding each other attractive - especially since it is just as easily perceived by children before they are sexually aware, and perception of certain aspects of beauty doesn't seem to vary with gender, sexuality or culture.

So what is the evolutionary advantage to us in enjoying beauty in things that follow some underlying maths? This seems, if anything, even more weird.

I do not understand why enjoying beauty in things that follow some underlining maths needs to have an evolutionary advantage.

Similarly, is anyone aware if any work has been done on why we have a sense of humour?

Yes.

Does the fact we can find concepts and ideas beautiful help you understand at all?

Please note I am not attempting to argue that "beauty exists therefore God exists", I am actually curious about the question.

I would hope not, due to the logical conclusion that soon follows when the fit of laughing ceases.