Yeah, I'm gonna stir the pot as I always do in these sort of questions but it's just being honest
The goal of a composition is the same of all art: To create something beautiful and great. Rivaling nature and worthy of a God.
A secondary goal would be to musically embody some sort of thematic concept or environment. For example, if my goal is to write a sweeping, romantic desert piece, but it fails to convince the listener of this idea, then it has failed in this regard.
It's only excusable to fail at the secondary goal provided your not writing to accompany a film or something, but never the first.
Everyone who knows me knows that I not only don't accept, but am openly hostile to modernist conceptions of music, art and architecture which instead argue that the goal of these things is to "say something"; i.e., propagandize. Abstract conceptualism which uses political or philosophical "meaning" allegedly reflected by the piece as being more important than the piece itself and certainly more than the actual quality or aesthetics since those things are rejected as "oppressive" by modernists.
Which leads me to...
Because they're lying. What they were hoping for is that you would gush over their work, but instead pointed out flaws with it.
Why do they lash out? Simple, and this is the most taboo subject in the world right now: They're totally brainwashed by the "equality" religion. They simply cannot accept that they are not as good as someone else in some regard.
Everywhere you go, at least in the West today, everyone believes in "equality" in all things and people and that everyone is a blank-slate individual all equally-capable.
People who have not been indoctrinated by this simply do not believe it. It is not an organic belief. Which is why no person who hasn't been indoctrinated by it genuinely believes that Kandinsky was good a painter as H.J. Draper.
When someone gets mad at you pointing out that their melody is incoherent, doesn't work with their chord choices, and the lines are too stale because of lack of vibrato or whatever, the reason they are made is because you have claimed that their piece is inferior to others. That they are not "equal".
and you know what? People who are superior, and know that they are, tend to be the most humble and helpful.
Musicians and composers who were and are better than I, were only ever helpful to me but every guy making avant-garde noise music that sounded dated in '95 is SURE that I'm a d*ck who doesn't know what he's talking about when I say that music that sounds musical is indeed better than music that doesn't.
I have never had a beautiful woman, or attractive people in general be rude to me — but every wicked witch of the west happened to also look like one...and insist they should be "models" too.
I think you get the idea. It's the people who don't believe in equality that allow themselves to become the best can be at any given thing, because if you believe everything is equal by default, then you do not believe there is room for improvement...and improve they never shall.