This Adagietto is the fourth movement (of five) and may be Mahler's most famous composition. Dominated by strings alone (with harp) it is the epitome of romance. Apparently it is a love song to his future wife Alma Schindler and a letter was found, from Alma to Willem Mengelberg, with a small poem from Gustav Mahler:
Wie ich dich liebe, Du meine Sonne,
ich kann mit Worten Dir's nicht sagen.
Nur meine Sehnsucht kann ich Dir klagen und meine Liebe.
How much I love you, you my sun,
I cannot tell you that with words.
I can only lament to you my longing and love.
This piece has inspired many choreographers to create ballets and has also been used as the principal theme for a few movies, including Death in Venice by Luchino Visconti.
If someone had written that for me I'd definitely say yes to the marriage proposal. Lucky Alma. Although lucky me: I'm playing this symphony for the first time in a couple of weeks. :)