What Makes Ian Somerhalder and Nina Dobrev's Relationship So Great?

How do they stay so happily in love while also working together so closely as Damon Salvatore and Elena Gilbert on TVD?
Nina’s first tip for having a great relationship? Date Ian! Just kidding. But The Vampire Diaries actress did share some dating advice with On Air With Ryan Seacrest.
“Make sure you’re smiling! People should be with the person who makes them smile. The minute you’re not, you shouldn’t be with them,” says Nina.
That’s a good point — do you think it applies to what’s going on with what's happening on The Vampire Diaries right now?