Regulus wasn't too fond of thunderstorms. As they triggered old memories of his mother and father. He's glad he got away from them but the memories are still there.
After he finished 4th year and Sirius finished 5th, they decided to run away.
After a day of Sirius owling James, they settled that the two brothers would run away to the Potter's house.That night around 1am, they packed their necessities and took off on their brooms headed to Sirius' best friends house. When they arrived, James was outside and said he had already told his parents. So he showed the other two to their sperate rooms where they would be spending the rest of the summer and went to his own room.
Regulus tended to spend as much time away from Sirius and James as possible and keep himself busy in the library.
But of course, Sirius and James would always find a way to annoy him on a daily basis. They would put gross seasonings in his coffee, or put worms in his bed. Which leads in the younger boy to yell at them. Which of course they only laughed at.
They weren't just bullies though, they had their moments. James has a pool in his back yard so they would all spend a night each week to go out and spend time together messing around in the pool.
And when Sirius was asleep, Regulus and James would cover him in shaving cream and feathers.
James and Regulus would spend evenings together on James' bedroom balcony and occasionally go on evening walks together.
Sirius of course noticed this. He always knew James had a thing for Regulus. And based on how Regulus looks at James, the feeling was probably mutual.
It's just a matter of time before one of them makes a move. But no one would have thought a thunderstorm would be the one to push them.

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comfort - Jegulus short story
FanfictionReg is afraid of thunderstorms, and James comforts him