Even when compared to other dragon pokemon, who are almost uniformly proud, territorial, and standoffish, Sliggoo are extremely solitary creatures. Sliggoo are blind from the moment of evolution and must rely on echolocation, but should two Sliggoo meet, their sounds will interfere with one another, leaving both all but unable to perceive the world around them.
In spite of this, every few years or so - whenever a sufficiently powerful rainstorm comes - the Sliggoo leave their caves, gather into a massive horde and chase after the clouds. These Sliggoo are in every case nearing evolution, but Goodra's regained eyes are a sudden transformation, not a gradual process; although Sliggoo can use the sound of the rain and one another's voices to keep up with the storm, they are notoriously bad at avoiding obstacles. Yet these obstacles are more often than not a blessing in disguise. Sliggoo's slimy and porous bodies make them appear more wounded than they are, and when any straggling Sliggoo defeats the inevitable challengers, they often find themselves to be still close enough to the rain to evolve into a Goodra with a significant territory after the rest of the horde has moved on.
It is unknown to science (and often frustrating for their trainers) why the Sliggoo require a natural rainstorm to evolve. The idea that it distributes territory evenly has been suggested, but the portion of the horde which catches the clouds and evolves vastly outnumbers those who fall into obstacles. Some have suggested its body takes a cocoon-like form which must be washed off, but in truth a Sliggoo's skin treats water no differently than a Goodra's. Many prefer the explanation of legend; that they depend on rain because Sliggoo, with their liquid bodies, always wanted to be water pokemon.