odd...

Every so often, I stop listening to the new stuff I've found and go back to a few tried and true bands like Stars, Death Cab, and Broken Social Scene. And every time I do this, I wonder why I feel so much more connected to these bands than the bands I've discovered more recently. I don't know why, but it seems like the older stuff has much more significance in my life than new material. Now undoubtedly some of this is explained by the fact that I've been listening to Stars, Death Cab, etc for a lot longer than anything else, and a lot of other bands that I started at about the same time sit unplayed in my library.

So part of it is because these are the best bands that have stood the test of time. However I also think that because these were among the FIRST bands I listened to, they have additional significance. There is something very romantic and mysterious about Stars, and not solely because of their music. So as more and more bands are discovered, there is less "mystery" about them, because you can compare them to stuff you've listened to previously. While this is useful for classifying music and finding similar bands, it decreases the uniqueness of new music. But when you find something that is truly different, however, you can really get into it, because of the novelty. Of course, this novelty makes it harder to appreciate the music at first, but once past that, I think your enthusiasm for the music is higher than if it was more similar to previous artists. A prime example of this process is Animal Collective. It was so far removed from the less experimental indie I've been listening to that it took a few weeks for me to even play a few tracks. Then I suddenly discovered that it was a lot of fun, and now it's something like my 6th most played artist.

So if you want a mathematical formula,
enthusiasm = (talent of the band) + (length of time you've been listening) + (novelty of the band)
where talent is generally constant, perhaps rising with time, time is obviously increasing, and novelty is inversely proportional to the number of bands you've listened to that are similar to the new artist.
and that's only if you're absolutely desperate for a formula, since my variables are vague and only give general trends.