Here's why:

* Size and Mass: Both Pluto and Eris are dwarf planets with similar sizes and masses. Eris is slightly larger and more massive than Pluto.

* Composition: They are both composed primarily of rock and ice.

* Orbit: Both have highly elliptical and tilted orbits that take them far from the Sun.

* Surface Features: Both show evidence of nitrogen ice on their surfaces, as well as evidence of geological activity in the past.

While other dwarf planets like Makemake and Haumea share some similarities with Pluto, Eris is the closest in terms of its overall characteristics.