Love is a powerful emotion, capable of destroying the strongest men. Is it any wonder, then, that an empath would fear it?
Hisoka gets caught up in Tsuzuki's emotions on a daily basis. The onder shinigami feels so deeply, so strongly. His emotions floor Hisoka with their strength, but he's so fickle. The love lasts no longer than the pie. He even loved Hijiri, but as soon as the human was back out of their lives...gone.
And that, even more than love, frightens Hisoka. Because he loves Tsuzuki, but if Tsuzuki ever realised, it would be all over.
Wouldn't it?