The composer is George Frederick Handel.
The melody for the song began its life as an Asian dance in his 1705 opera Almira. He then used it as an aria in the 1707 oratorio ‘Il trionfo del tempo e del disinganno’, with a different text and name, “Lascia la spina”. Handel then recycled the melody again to the now famous Lascia ch’io pianga in his 1711 opera Rinaldo. This rendition was a major triumph for Handel, and it is this version which we know and love today that has become so popular.
I think the baroque instrumentation and melody summs up people’s feelings in a way only music, and in particular song can express. The deep sadness and melancholy of the aria have proved timeless in its beauty.
The character of Handel’s opera is bemoaning and mourning her freedom, sad to be away from her lover and trapped seemingly beyond help. This sadness easily translates to the sadness associated with the loss of a loved one.