Yusuke's smiles, laughs and happiness being contagious and influencing Hiei's moods is so canon.
Yusuke feeling melancholic and depressed after he witnessed Toguro kill Kuwabara was also the same contagious emotion for those at the tournament when it happened. They could feel it from his very energy.
So, Hiei bears both his and Yusuke's grief when Kuwabara dies for real. Yusuke's emotions seep into Hiei's skin and nerves like cotton or gauze absorbing blood.







