Tobirama assumes all Uchiha are like Izuna and Madara because they are the ones he interacts with the most. That's why he doesn't go out of his way to interact with anyone else from the Uchiha Clan. He doesn't want more headache.
So when he has to interact with Hikaku, because Madara is out of the village and Izuna can't be arsed, he is on high alert. Hikaku is older than Madara, probably as temperamental as him, Tobirama assumes or hates the Senju even more than Izuna and it makes him sharingan happy, like Izuna.
To his utter surprise, Hikaku is a saint compared to the other two. No sharingan in sight, no angry shouting, no raised chakra, just an even tone and carefully chosen words.
Tobirama comes out of the meeting like a different man. That doesn't change his opinion on Izuna and Madara being the worst, but if there's one pleasant Uchiha then there should be more, correct?
Tobirama is willing to experiment and strike up a conversation with random Uchiha Clan members and mentally fill out a checklist of how close that person is to either Madara and Izuna or Hikaku.
Meanwhile Izuna is fuming because Hikaku ruined his fun and now he has to be nicer to Tobirama to get his attention. Tobirama's been avoiding him instead of putting up with him like he used to.