They both have a very narrow circle of friends or acquaintances because of who they are and they both have difficulties forming any sort of meaningful connections with other people.
They’ve both turned out odd despite normal upbringings.
They both have dysfunctional siblings that have played a part in who they are now and vice versa.
They’re both on the fringes of society, by default or design.
Each is a barometer of ‘normal’ for the other; Shizuo delineates monsters/humans for Izaya, and Izaya defines the pinnacle of annoyance for Shizuo.
They both have issues with control; Izaya lives by it, and Shizuo thinks his world would be better if he had more of it.
They both have issues with protecting; Izaya’s all about self-protection and self-preservation and Shizuo’s all about protecting other people from himself.
They both have issues with esteem and an equally warped perception of it; Izaya has an inflated sense of self and Shizuo has a lower-than-dirt sense of self, and neither are accurate, and as a result…:
They both have trust issues; Izaya doesn’t trust anyone else, and Shizuo doesn’t trust himself.