Disclaimer: I do not own the canon, I merely enjoy it.
Starring: Flash/Batman
Author's Note: While watching Crisis on Two Earths, the dynamic of the two is very apparent.(I don't know, possible slash here?) Also, I'm trying to seperate Batman and Flash from Bruce and Wally. It is often speculated - and I agree- that Bruce Wayne is the costume and Batman is the real thing, Wally on the other hand, I believe is himself in and out of the spandex.
Summary: Flash is impulsive and lively, it's his nature. It is not something Batman holds against him, rather, it is something he tries to protect.
"You really think he'd risk your life if it wasn't sure?" - Wonder Woman
"Well, maybe. I don't think he likes me very much." - Flash
(Justice League: Crisis on Two Earths)
Bruce likes Wally. Really, he does. There is something child-like in the way that he talks to others (villains or not), the innocent way in which he seeks to avoid work but fully supports cooperation and negotiation. He doesn't like to see others hurt and is often frustrated at how his own impulsive nature sometimes allows bad things to happen to others. His apparent distrust of Batman however, is not something Bruce holds against him.
Batman is the polar opposite of everything the Flash embodies. Batman cannot afford to be impulsive, or lively or even always strive for negotiation. He has to be broody, he has to carry the darkest villains – not to say that the Flash does not have a dangerous Rogue Gallery of his own, but Batman deals with the psychotics. He is the only one who can.
So while Batman himself must be secretive and restrictive, there is not reason for Flash to ever be like him. Batman is willing to sacrifice others to protect Wally so that he maintains all that he is and will be, and sleeps easy with his decision.
Bruce himself cannnot live, but there is not reason Wally should not.