Lavender is known for quiet calm, for soothing those that really need to take a break. It's like a gentle, refreshing gift.
It's not that Yona seems particularly stressed, but that they see a lot of unpleasant things and walk terribly far on feet that easily get tired, and besides Yun just really wants to give her a gift the only real way he knows how to.
"Here, Yona." He hands her a warm cup, "It's hot." He cautions.
"Thank you, Yun!" Yona's face is lit up and even turned a little pink in her joy.
"No problem." At her blushing, Yun's face even turns pink. "It's a lavender tea. It's a calming drink." He offers a little embarrassed now.
"You're really smart." Yona answers genuinely, the honest compliment easily falling from her lips.
"I guess so." Yun does look away now; it's harder to admit after a compliment than it would be otherwise.
Yona's face is still lit up by a wide smile as she finally takes a sip which leads to her complimenting Yun's ability to brew a good tea, that is not only soothing but absolutely delicious.