I too learned harmony singing at my mother's knee, or rather, at her side, in church. I remember being very young, and noticing that she wasn't singing the melody of the hymns, but was singing something that sounded very beautiful to my 6-year-old ears. So I just joined in and sang with her- I remember her startled glance and her smile- a tremendous incentive to listen more and try to figure out how music worked.
I've been singing harmony ever since! I never really knew what I was doing until I got a university degree in music- but even that didn't take the spontaneous joy from me when singing.