I'm with El Greko. I just can't help it! I sang in choirs from the age of 8, boy soprano (treble) at first and I soon loved to do those descant harmonies. Later as my voice broke I migrated through tenor to bass in choral work but naturally I'm in between (baritone). That sometimes gives me a problem singing along the melody line with many performers, so I just lapse into harmony quite naturally. Normally, by the second chorus of a song I'm hearing for the first time I'm away, and humming a harmony under my breath during the verse!

Having come back to folk music as a solo performer about 4 years ago, for the last 2 I've been singing in a duo with Christine Connolley (Moses here) and we fight over who will sing the harmony almost every time we learn a new song!