It can't rain forever.
This is based on the quote "Be strong; things will get better, it might be stormy now, but it can't rain forever".
Simon loved the sunshine. He loved the way it made everything look brighter, it could liven up even his worst days. He loved the way it just made everything look so alive, so happy, so...breathtaking. He loved walking around, feeling the sun on his skin and it made him feel like he belonged.
He knows really that it's not the sun that made him feel like this. Originally, it was Clary. They spend almost all of their time outside, always finding nice places to relax despite being in the city. He always associated the sunshine with Clary, she brightened up his days and gave him life, gave him a reason to live. He loved the way it shone on her hair, the way it made her beautiful eyes brighter. He loved the sun, and he loved her.
But that was before. Before he changed. Now he longs to feel the sun on his skin, to warm him up with its gentle touch. To breathe the life back into him, for now he felt he was wasting away. It's ironic, that now he can live forever, but he doesn't feel alive. He would do anything to walk in the sun again. Or maybe, it was just Clary he needed. The nights would be a lot more bearable if he spent them with her, he hadn't seen her in so long. He does see her occasionally, of course. But she's changed, she's not the same as she used to be. He wanted everything to be back the way it was originally, both Clary and the sunshine.
Now that he is confined to the night, he hates going outside. The night always seems to bring rain with it, something he never noticed before. The last few days, night after night, rain fell from the sky in heavy showers. He supposed it didn't matter really, as he could no longer feel the cold. But still, he hated the rain. It was the complete opposite to the sunshine he loved so much. And of Clary.
One night he finds himself sitting on his desk, staring out of the window, watching the rain fall. It was quite mesmerizing, really, now that he is taking the time to observe it. It was relaxing, watching the drops fall to the pavement, with that distinctive pitter-pattering sound. Amoungst the rain drops, he hears an echo of something his mother once said when he was hurt about Clary; Be strong; things will get better, it might be stormy now, but it can't rain forever.
Back then, he didn't know how true she was. Now, sitting in his back garden, Clary by his side, he feels so alive again. His sunshine is back.