The following article contains spoilers from Nightwing #91, on sale now.
Wally West has always been Dick Grayson's best friend and biggest supporter. He wants to see the man he admires so much have everything that he deserves in life. Not just a successful career and for him to live his dreams, but to also have a healthy, happy romantic life. In Nightwing #91 (by Tom Taylor, Geraldo Borges, Adriano Lucas, and Wes Abbot), he even tried to push Nightwing to pursue a more serious relationship with Barbara Gordon, citing that the two had so much in common.
The thing is, Nightwing isn't certain that his and Barbara's relationship is strong enough to survive such an attempt, or even if it's headed in that direction. The Flash believes in the opposite, that Nightwing and Oracle have what it takes to successfully build a life together. He may be letting his own relationship with his wife Linda bias his perspective though.
From the outside looking in, the Flash isn't wrong about the two. They share so much in common. Both have lost people and endured terrible hardship to get to where they are now. Nightwing has lost his parents and lost his memories for a time. Oracle has lost her brother, and the use of her legs at the hands of the Joker. Both know what it is to lose those they love and overcome any physical challenges sent their way. Both their worlds orbit each other, so it's not too much of a stretch to imagine them supporting one another through anything.
The problem with this is that neither seems certain that committing to a relationship is a good idea. A spark is there, that much they won't deny, but the two of them have a complicated past. They have skirted around the idea of being a couple for decades. In fact, in real-time, the two only entered their first relationship thirty years after their first official meeting as Robin and Batgirl.
There was even a brief period in which they were set to be married, but Barbara broke it off, believing that Dick needed time to figure out who he was as a person. Adding to the confusion of the on and off relationship are Dick's own many famous relationships outside of Barbara. The most prominent of course is Starfire, but they're even more on and off than Dick is with Barbara. Considering how many writers and fans like to see romances flung Dick's way, it would be hard to convince anyone to have him commit to a single relationship.
This seems to be something Wally is forgetting, and it might have something to do with his own marriage. For Wally, Linda Park has been the only person in the world for him. He has raced through the speed force for her and even sacrificed his own capacity for human emotion just to ensure that she regained her memories as well as their children. She has always been the sole focus of his affection. In contrast, Dick and Barbara have had other relationships that make them question whether they are right for one another.
Eventually, they will have to settle this will they/won't they question. As likely as it is that there are too many factors contributing to their parting, there is also just as much evidence saying they can work it out. No matter how bad things get or how messy their own relationship becomes as time passes, the two naturally are drawn back to one another with time. So while the Flash may not be looking at the whole picture of their history, he is right in saying that there is a foundation for them to build upon.