I think the reason they get along so well is because they're polar opposites. Yoongi needs Hoseok as the optimist to his own pessimistic outlook. Hoseok needs Yoongi as proof that he's needed, as a reassurance when in self doubt (which yoongi undoubtedly provides as someone who cares for the members mental and physical health), and in his own way sensing that Yoongi is mentally vulnerable- becoming his pillar of sorts.
