Roger Scruton has written an important article on modern art, one that I encourage you to read in full.
It begins with the question of what high art is for. Scruton observes that modern art is transgressive and aims to shock and confront. Traditional art was oriented more toward beauty.
What is impressive is the further development of these ideas by Scruton. Scruton argues that beauty in art did not exist just for aesthetic purposes but expresses a deeper experience of life, a sense of the sacred, that makes us feel at home in the world.
Modernists do not feel at home in the world, and therefore aim to desecrate: the mockery, the cultivation of ugliness and the moral transgression is aimed as a pre-emptive strike against the deeper experience of beauty referred to above.
If you do read the article, take a moment to compare the two paintings used to illustrate traditional and modernist art. The traditional painting, by Francesco Guardi, is described as "capturing the intimations of the eternal in the transient". The modern painting, by Otto Dix, is very different, being described as "wallowing in the base and the loveless" - an assessment that is difficult to disagree with.