A susceptibility to change; instability.
Up with the rocket, down with the stick: one emotional project replaced by another the instant the smallest thing went wrong. To Shaw at seven years old, her lability had been both her curse and her deep mystery – her cult. […] When things were good she couldn’t stop talking them up; the next day they had become too complex, and she left suddenly, and might not be heard from for months or even years, after which there was a postcard, a tearful but stiff reunion which inevitably prefixed another parting.
Definition from Wiktionary, Creative Commons Attribution/Share-Alike License (via Wordnik)