In the DC Universe, Superman is the one hero everyone trusts to help them in their darkest moment - everyone, that is, except the Flash, who has a chilling explanation for why his relationship with the Man of Steel is very different.
When he was the Flash, Barry Allen and Superman famously raced multiple times to see who was fastest. It was later explained that Barry limited his speed against Superman since the races were for charity, and his successor Wally West turned out to be an even faster speedster, placing him well out of Kal-El's league. Sadly, one villain who's always been able to keep up with Barry is Hunter Zolomon, aka Zoom, who became obsessed with subjecting the Fastest Man Alive to personal tragedy in order to 'improve' him as a hero.
In The Flash #200 by Geoff Johns and Scott Kolins, Zoom and Wally circle the globe in a furious battle, traveling so fast they are going around the world in less than a second. The Flash notes that during the battle, he keeps seeing his Justice League allies frozen in place, unable to help despite the incredible threat to everything he holds dear. Even Superman is pictured at a standstill, turning Flash into the one person in the world who can't hold out any hope that the Man of Steel might fly to his rescue while he's under attack.
This is a terrifying thought, as Superman is the savior of last resort for so many on Earth. Superman can be trusted to show up when a Justice League member badly needs his help. Even his arch enemy, Lex Luthor, can count on Superman to be there to save his life when in danger. When a villain proves too much to handle, Superman classically steps in to help take them down, but while Zoom certainly fits the bill, Flash is totally on his own with no chance of a last-minute reprieve. What's particularly dark about this is that due to Flash's speed, he's forced to see his Justice League allies again and again as he fights for the safety of his family.
It's bad enough that Flash can't count on Superman to help him, but it's made far more disturbing by the fact that he knows exactly where the Man of Steel is, and is even passing in and out of his sight line during the fight. Unfortunately, things are happening so quickly that Flash could even be killed in front of Superman and his Justice League ally would only know about it after it was too late. Superman has traveled across space, time, and dimensions to help those in need, but Flash's greatest conflicts are settled in a way he just can't match, meaning that the Fastest Man Alive lives in a world protected by a hero who is able to save everyone else but him.
In many ways, this underscores how noble and heroic Flash actually is. Even Batman can count on Superman in an emergency, and the Justice League was founded on the principle that the heroes will work together against major threats. Of course, whenever Superman or his other allies are in dire need, the Flash is there to help, despite knowing he can't expect the same in return.