Mab and Titania haven't spoke in like 1000 years, and it was over a man whom Jim said didn't survive. We also know that when things got bad in the wizard world both Queen's died resulting in the rise of Mab, and Titania. I'm wondering if they were in love with a Starborn, possibly even Merlin, and that person brought about the end of the previous Queen's somehow either through action, or inaction.
This could explain Titania's and Mab's feelings towards Harry, a Starborn as they had trusted, and had feelings for the previous one. Jim described Titania being very nervous of Harry, and Mab keeping her enemies close.