One of my favorite things about music is the way you drift in and out of levels of attention - and how lyrics reveal themselves slowly, a different line will pop out. Or you'll notice some new feature of the arrangement or instrumentation. Outside the reviewing situation, I tend not to listen to songs or rap tunes and follow the lyrics focusedly all the way through - different bits will leap out - the whole idea of a song as a story is a bit foreign to me (although obviously there are exceptions, where there's a narrative pay off, or a sense of build towards something)
"Sessionability" relates to this thing I think is fairly unique about popular music which is repeatability. There are films that you like to watch many times, usually over the course of a life, but sometimes in the first weeks of release, you'll hear of people going multiple times. But almost no one, I should think, would watch a film again immediately after watching it. But that is very common with pop songs - you hear it, you want to hear it again immediately, and possibly again, and again. Or with whole albums.
It's a fairly unique thing with pop music. The way that there's repeatability that doesn't wear out the pleasure. It delivers the hit again and again. (eventually you'll get tired or wish for a bit of variety, but it might not be for a long, long time). In a way it works more like a drug than a dose of culture.
You wouldn't do that with a book. Well, I seem to remember one of my kids finishing the latest Harry Potter then immediately reading it again from the start, reading a little slower to savour it. But generally not... generally it'd be a matter of years before rereading a favorite book, which is a whole unique set of pleasures in itself, almost voluptuous in the sinful sense of not reading all the other books you ought to be reading but indulging this nostalgic delight)
Perhaps it's just to do with the unit-size of the aesthetic object. Songs being in the 2 minute to 6 minute range. Perhaps if films were typically 10 minutes long then people would watch them over and over. But somehow I doubt it.
With other kinds of music, where the duration is longer than in pop, it's probably got less repeatability. Although classical music has kind of been pop-ified, with the most catchy bits pulled out of their context and played on light-classical radio stations.
And maybe it's the non-narrative, non-story nature of most pop songs - they are loops of feeling, emotions freezeframed, or units of action smaller and shorter than a narrative, perhaps more like a vignette or scene - that also contributes to this repeatability. If songs had plots like films, at a certain point the suspense element or resolution element would get worn out.
(Mind you, there are songs like Harry Chapin "Cats in the Cradle" which has an emotional twist in the tale does seem to always work, never wears out. Or story songs like Pulp's "Common People" that you never tire of the narrative. Perhaps the analogy is the anecdote that gets wheeled out over and again, but no one gets fed up with hearing (or telling).
