Open strings

The four strings of the violin are tuned G, D, A, E and, the strings of the viola and cello, in C, G, D, A (an octave apart). When played unstopped by the player's fingers, these (open) strings sound more sonorous. The four notes G, D, A, E are the notes of the D major and G major. This is why these keys are featured more frequently in music for violin (three of Mozart's five violin concertos and Beethoven's only violin concerto are in these keys).