Ed loved rubbing in Roy's weakness. Taunting him, till the older alchemist grew annoyed with him.
Ed knew all about rain and how it affected Roy Mustang. He especially liked the word first lieutenant saved for the man. Useless.
Honestly, Roy had thought that Ed had matured some and was past the whole name-calling thing. Apparently not perhaps, Ed was still being childish after all these years.
No matter how much Ed rubbed it in, Roy never left. He just took it all in. because Roy knew that Ed would apologize for it later.
In his own unique way of course.