I was thinking about this last night.
I went to a concert last night, which I enjoyed tremendously--but I wouldn't go again on this tour. The same holds true for a concert I went to two months ago. Had a good time, thanks. See you next time around. Maybe.
With FM--I went to 10 SYW shows (which isn't even all that many according to the bylaws of the FM mania handbook
A couple of them were either the next night or with a night in-between.
I ADORE Fleetwood Mac. Not just like. Not just love. ADORE. As people, as musicians and as performers. And, even though the setlist doesn't change very much--each night is different to me. I think they are just that expressive. You know when one of them is a little tired, a little pissy, or feeling completely "on." It's written all over them and I love witnessing as much as I feel I can financially afford. And when they are all "on," it's a sight to behold.
But I don't necessarily think that it's something that can be explained by anyone. I think you can only "get it" when you experience it with a singer or a band or someone you do want to see multiple times yourself. It's just some odd cosmic connection or something;-)