As much as it pains him to admit it, rain reminds him of his lost brother. If asked about the comparison, he would explain that Loki always shined the brightest in the gloom of a rainy day. His younger brother became the sun on those days, radiating a warmth that never existed on any other day. He had a joy that couldn't be rivaled. And Thor coveted those days because he wanted nothing more for his brother than to see him so happy.
Without Loki, rainy days began to weigh heavily on his heart. He would think too long on things that used to be, or could have been. He found himself slowly drifting into an uncharacteristic sadness on the days when the sky was gray and crying. Sometimes, he even found himself crying with it.
Until he met Jane.
With Jane Foster, Thor found his light again because Jane loved rainy days just as much as Loki had. She glowed just as brightly and smiled even more. On rainy days, Jane threw herself into her research with more vigor than Thor had seen in Volstagg, throwing his-self into a feast. On rainy days, Jane Foster was simply more alive. And that was the exact same way Loki had been in their youth.
It was those days that Thor chose to spend with Jane more than any other. No matter the date, if it was raining, Thor was with Jane. Most days, that meant sitting on a stool in Jane's lab, watching her work. Often, mellow music played softly, only just loud enough to be heard over the gentle patter of the rain.
When the rain was too hard and interfered with Jane's work, they would sit, curled up on a wide suede sofa, and listen to the same mellow music. They would whisper stories about their childhoods, laugh gently together. And Thor, whose only volume on any normal day was sonic boom, was quiet and light as the flutter of a butterfly's wings. Because he couldn't, for the life of him, bear to disrupt the peace that was between them. He wouldn't distract from her vitality.
And so they lay together, murmuring stories in the gray light. And for all the world, Thor could not think of one thing that filled him with more joy than lying on that sofa on those tranquil days.