I might be missing the point totally, so can someone please clarify it for me??
It's mostly theory. *shrugs*
-Tobi and Obito have similar hair,
-both attitudes are far from regular shinobi attitudes,
-Tobi's mask's eyehole is employed for the right side of his face -the side of the face that Obito's face was trapped by the boulder.
To a lesser extent, some people would also say
-the similarities in their names,
-Kishimoto's reluctance to kill characters,
and a small list of other theories that'd branch out further...