Them being utterly perfect
On a totally unrelated note - Remember the Lucien from ACOTAR? The sassy little fireboy? One with always a sarcastic comment for everything? Yeah I want him. And I want him back now.
I want Elucien together because he actually respected her wishes. When she refused to see him, he maintained a respectable distance. He asked Feyre about her PERSONALITY.
This might not seem like much to others but rarely do we see people look beyond Elain's beauty. He thinks that she's the most beautiful girl he has ever seen ! He literally broke Hybern's restraining spell on him when Elain was chucked in the cauldron and then dumped on the floor. Tamlin remained restrained, but Lucien got free and went to pick Elain up.
He wouldn't coddle her like the others do. He actually sat and listened to what Elain had to say. Even when she asked how if he could hear her heartbeat (that would honestly make me run for the hills). I can just imagine her run her fingers across his scars and call him the most beautiful male she has ever seen. Also if Elucien is together you would get to see both Elain's and Lucien's personality shine through.