Ezra's bond with Sabine is practically immediate in "Star Wars Rebels" since they're the only two teenage members of the Ghost Crew. At first Ezra harbors a crush on Sabine, which she doesn't reciprocate but nevertheless seems to find endearing. Soon the notion of romance takes a backseat to a close companionship as they help each other through plenty of requisite teen angst concerning their respective roles in the rebellion.
When Ezra makes his final sacrifice in "Rebels," he tasks Sabine with keeping Lothal safe. In fact, Sabine's tower-like home in "Ahsoka" was once Ezra's. So, in Episode 1 when Sabine pulls out a recording Ezra made her before leaving, it's indicative of the fact that, even 10-plus years later, she's not just still thinking about him but situating much of her life around his memory.
Furthermore, in the final sequence of "Rebels," Sabine and Ahsoka leave together to find Ezra. Between then and "Ahsoka" they apparently failed at their mission and had some sort of falling out. Ezra, then, may not be physically present as of "Ahsoka" Episode 1, but his absence colors both Sabine's daily life and her dynamic with her Jedi mentor considerably.
