Learning music by ear.?

you pick it up after playing for a while. I have played violin 6 years and it took me a shorter time then most because, i don't know if this happens to everyone, but in my head there is like a constant starting note. Like if i am going to sing a scale i always automatically start there. I don't know if that constant note developed or was always there. Once you figure out what note that is, then its just a matter of knowing how spaced out notes are and how many steps above or below your starting note the other note is. for example, my starting note is always D, so if i here a note that is 2 steps below, i can tell it is B.