  We introduce a new proposal for the onset of cosmic acceleration based on
mass-varying neutrinos. When massive neutrinos become nonrelativistic, the
$Z_2$ symmetry breaks, and the quintessence potential becomes positive from its
initially zero value. This positive potential behaves like a cosmological
constant at the present era and drives the Universe's acceleration during the
slow roll evolution of the quintessence. In contrast to $\Lambda$CDM model, the
dark energy in our model is dynamical, and the acceleration is not persistent.
Contrary to some of the previous models of dark energy with mass-varying
neutrinos, we do not use the adiabaticity condition which leads to instability.
