WHY "After the rain there's always Sunshine"?

I chose this title thinking of Meroko and Izumi's relation.
Arina-sensei declared that the rain represents Meroko, so since she's like the sun for Izumi, I put the two elements together.
Meroko and Izumi's life were sad and full of pain, especially the Izumi's one, because she thought that nobody loved him. In death he meets Meroko and they fall in love, even if in a first they don't understand that feeling.

Yes, Meroko is the rain and the reasons are her past and her chatarsys, but after death she find the peace and Izumi and her finally can stay together with a smile.
The darkness left their heart and now they're happy, so I think that it's true that after the rain, there's always sunshine. :)