When speaking of modern music, it may be the complexity of rhythm or harmony of the piece in question, a lack of memorable melodies or it may be a simplicity in the rules implicitly underlying the piece, which only makes the freeness of the music seem complex to the listener's higher faculties when they try to analyze it. Just as a thing can be understood intuitively or felt strongly to be so which the thinking, rational part of the mind finds impossible to prove, or can only justify after much difficulty. Some point to Debussy's L'après midi d'un faune as the first usher of 20th Century music.
In a way it is pointless to try to write words on music like this, but here goes anyway. It doesn't really help to read glib selective quotations from even the composer describing the music, sometimes in a single word, "tragic," "fate," "Heldenmord" fail to do justice while missweighing one idea, like a greedy fruit grocer. The Mahlers deep and checkered feelings about his Sixth Symphony are clearer from this quotation from Alma Mahler's memoirs, even if it does sound ambiguous or contradictory at one level: