Eridan has always found the rain interesting. Whenever it rained, he would stop whatever he was doing and stare at the nearly invisible drops of water that hit the sand outside his hive. Any kind of storm with rain in it is all that Eridan has seen, but he enjoys any of them. The light rainstorms are perfect to stand in and to just stare into the sky. The rain that pounds on his hive relentlessly helps this Ampora sleep (for some time he only sleeps in that kind of weather). The thunder and lightning of the most feared storms help our Eridan create his little doomsday plans.
He grew up with these storms, and he often wondered why no one else liked them like how he did. Sometimes, when a certain kind of storm would come, that one storm that comes once a sleep, Eridan gets onto his lusus and rides around during that storm, angering his lusus.
As everyone knows, Eridan hasn't had the best life. Everyone hates him, and the only person he wanted to be matesprits with rejected him. On lowaa (Land of Wrath and Angels), the only thing he missed about Alternia is the rain. Each day, he would get up and, if he wasn't planning on dueling with Sollux, go outside with the only hope he has left anymore, the hope for rain. It never came, and he missed it more than anything else from Alternia.
As time passed by, and the trolls had to move to the meteor, Eridan still only wished for one thing, and that was for a rain storm. Even it was the small kinds that only lasted for a few minutes and didn't produce a lot of rain, he still wanted one badly. The closest thing to rain he could get was a shower, and that didn't last very long.
Soon, when the trolls met the kids, and Eridan found out that Rose's planet was the loral (Land of Rain and Light), he would purposely stay on Rose's to just watch the rain. He had to converse with Rose about her "fake" magic, but that is understandable for Eridan to do.
After Eridan died, and he started to live within the dream bubbles, he searched around for somewhere with rain. After days and days of wandering around the dream bubbles, coming across many of the dancestors and other versions of his friends and himself, Eridan finds the somewhere where it constantly rains. He stays there, alone, but he doesn't care, because he can now for the rest of eternity listen to the rain falling all around him.