Berg was a student of Arnold Schoenberg and came to prominence with compositions using the atonalism of that school. He incorporated chromaticism and an absence of tonality into his compositions with the complete facility, if not to public acclaim. His creativity was interrupted by World War 1, during which he served in the Austrian Army. He returned to composition as a champion of modern music, with his opera Wozzeck (1923) bringing both fame and notoriety. He died of blood poisoning in 1935.
Over the past century, dissonance increased in the compositions of serious music to a point where the semitones had equal value, which is harmonically a kind of wall. Berg was an early innovator. However, if when strictly followed such serialism reaches an ultimate dissonance that effectively sees off melody and harmony as emotional and structural entities, that still leaves elements around form, dynamics and rhythm for the purposes of expression, and these together with adroit note selection prove to be surprisingly potent for articulation and cohesion.
As well, Berg's writing is rarely purely atonal. In fact, the integration of consonant elements is one of the music's most alluring features. It would be so easy, one feels, for melodic material to coagulate the mix, but in his hands, the very opposite is generated, an increased clarity of mood. The music remains consistent, and the incorporation of (often only relatively) thematic material, if often arresting after so much dissonance, doesn't always mean less intensity or gloom. It is simply effective.
Having said that, it can hardly be denied that the features of atonality (dissonance, clashing semitones, inharmonic bass) give it special suitability to express dark outlooks, and Berg is the author of Wozzeck and Lulu, no downtown musicals. With it, he discovered a way to express what he wanted to.
There is in Berg's compositions the kind of inevitability that all good art has, and difficult as that is to accomplish without melody he is reported to have worked with enthusiasm. Musical lines flourish despite the atonality, and one hears a rare gift for harmony. The complexities appear natural, not contrived. With Berg, seconds are often the glue that thirds were for preceding composers. The more abstract nature of the harmonies allows rhythmic abstractions (rubato, cross-rhythms, syncopation, motif rhythms etc.) to be readily assimilated; these are pervasive features, finely crafted, and add cohesion. Unlike music based on conventional harmonies rubato playing is often suitable.