or is it just up to someone's taste?
According to Pythagoras, it's up to maths.
P.S: I mean about people's physiological beauty.
Everything is beauty! The fact that we are here and alive is beauty! Being human is beauty! And all humans are beautiful! If you're not seeing it, you're missin it!
Some people are regarded beautiful by just about everyone while with other people it's definately in the eye of the beholder. While some animals are regarded beautiful by most people, again there are others where it is most certainly in the eye of the beholder.
As far as people are concerned, it probably is a matter of mathmatics. For most of us the left and right side of our faces are slightly different, but the few whose faces are symetrical - left and right identcal - are considered beautiful. George Clooney is one such person.
A lot of it comes down to personal taste. While most of us regard flowers as being beautiful, we all have different favourites.
To find what beauty is, you should find things that are beautiful in the abstract: colors, lines, shapes, et cetera and look for them to determine what in the real world is beautiful. Without doing that, you are just relying on biological hardwiring to determine what is beautiful, without ever actually making the decision yourself.
But, since which abstract forms one finds beautiful are different from person to person, beauty is indeed in the eye of the beholder. Perhaps if one could learn to find all possible forms beautiful, then everything would be beautiful, as was said earlier.