  The problem of the cosmic coincidence is a longstanding puzzle. This
conundrum may be solved by introducing a coupling between the two dark sectors.
In this Letter, we study two cases of the coupled quintessence scenario. $(a)$
Assume that the mass of dark matter particles depends exponentially on the
scalar field associated to dark energy and meanwhile the scalar field evolves
in an exponential potential; $(b)$ Assume that the mass of dark matter
particles depends on a power law function of the scalar field and meanwhile the
scalar field evolves in a power law potential. Since the dynamics of this
system is dominated by an attractor solution, the mass of dark matter particles
is forced to change with time as to ensure that the ratio between the energy
densities of dark matter and dark energy becomes a constant at late times, and
one thus solve the cosmic coincidence problem naturally. We perform a
statefinder diagnostic to both cases of this coupled quintessence scenario. It
is shown that the evolving trajectory of this scenario in the $s-r$ diagram is
quite different from those of other dark energy models.
