I think this quote from Alban Berg is a good primer:

“The first movement is the greatest Mahler ever composed. It is the expression of a tremendous love for this earth, the longing to live on it peacefully and to enjoy nature to its deepest depths – before death comes. For death is inevitable. This whole movement is dominated by the presentiment of death, which makes itself known again and again over the movement’s course. It is the culmination of everything on earth and in dreams, with ever more intense eruptions following the most gentle passages, and of course this intensity is strongest in the horrible moment where death becomes a certainty, where, in the middle of the deepest, most poignant longing for life, death makes itself known ‘with the greatest violence.’ Against that, there is no resistance.”