I think I'm beginning to understand what motivated this album. The obvious background, the one you can easily read up on, is that Lust for Life was recorded shortly after the conclusion of the tour for The Idiot and once again was a collaboration between Iggy and Bowie, who was in charge of virtually all the music (and whose background singing can be heard very clearly on "Tonight" - in fact, volume aside his voice might as well be the real chorus). Iggy's job was to figure out the lyrics, since "Bowie was [...] particularly impressed with Iggy's way with words", as told by Hugo Wilcken in his 33⅓ on Low. Where you can also find a quote by Iggy saying that he tried to increase his contribution to Lust for Life compared to The Idiot, so that it wouldn't end up a Bowie album in disguise.
So what is it about? I think I found the answer. Or at least I have a hunch. Well-founded speculation, if you will. Again there's an obvious answer: Of course it's about a self-destructive addict lifestyle. It's an Iggy Pop album after all. But I suspect another catalyst was Iggy turning 30 just before the recording sessions started. The big 0. It can be quite depressing. Take it from someone who suffered bouts of death anxiety since adolescence and who stopped celebrating his own birthdays at the age of sixteen. I'm not 30 yet, but just a couple of years away. Half of my twenties is over and already I catch myself always glancing at biographical data when perusing Wikipedia. At what age did X have his/her big breakthrough? How many years did they have left at that point? Oh, hot new artist Y is that many years younger than me...?
That is the origin of the "lust for life" that burned in Iggy's heart. At his lowest point, he had been given a second wind by David Bowie, musically on an absolute tear at this point while privately battling his own demons. And Iggy had to wonder how much of this new life was left in the body he had wrecked for years (serendipitously more than his fellow Stooges and even Bowie). This leads us to the fetishizing of youth a secondary theme on the album. Of course it's all over "Sixteen" ("I must be hungry, cause I go crazy [...] I know that's not normal"), then the rest is predominantly marked by thoughts of morbidity and mortality until we get to "Fall in Love With Me", where Iggy not only essentially turns the song title into a demand instead of a plea but also constantly, half-delirously stresses the youth of his object of desire.
Lust for life? Greed for life. Iggy already has a rock star career and every imaginable drug trip behind him, and now he wants life. Even if that means he has to feed off others. Hungering to preserve his young punk self, coveting every experience, the idiot turning into a vampire under the guidance of coven leader David Bowie.