Sehun love Luhan a lot, more than whole werld actually. Luhan gives him that tingle, and when it is Christmas Luhan jingle (he a reindeer for santa) 

He try to replace the deer but Luhan manages to creep back into all his thoughts. I'm creepin in ur heart babe  

Exo's maknae feels sad because Luhan is a thousand miles away, and there is no one who can make him smile brighter than Luhan can.