Because Lan Wangji is competent and collected and intimidating.

And he's now just standing naked in front of him. At least he isn't the first person he's been naked in front of?

Lan Wangji walked into the bathroom. "What temperature do you like your baths?"

"...Medium temperature? Wait, that's not useful. Um. Warmer than blood warm but not so hot it burns?"

Lan Wangji intends to sit here carefully adjusting the water temperature until it is perfect.

Wen Ning is being so good and assertive by waiting till they find a temperature he actually likes instead of panicking and picking the first one. 

Then he can get inside the bath!

Getting into a bath without using your hands is awkward, but he manages to get in without falling over, so that's a win. 

As soon as Lan Wangji sees that it's awkward he grabs Wen Ning and tries to guide him him.

He is being so well behaved and is letting himself be helped. And he is being so good and is having no reaction to Lan Wangji's hand on his arm.

Lan Wangi settles him in the bath and begins to very carefully soap him. He gently lathers up Wen Ning's chest and arms.

Wen Ning is doing his best to make it easy to do that. 

Lan Wangji has very nice hands.