Key to amazing relationships
The characters must KNOW each other-
I love Elizabeth and Darcy in Jane Austen's Pride and Prejudice because how well they grow to know each other. Darcy is rich. He could easily have gone to Elizabeth and said "here's a mansion. Love me?" He knows that won't make her love him. He knows how much she values her family which when he *insert spoiler* helps Lydia *ends spoiler*, it means so much more than a house.
The characters must TRUST each other-
I love Adrian and Sydney in Richelle Mead's Bloodlines series because of a gazillion reasons: the main one being of how much they believe in each other. Adrian is a little bit crazy. Everyone kinda chalks up his actions as him just being his usual crazy self. Sydney trusts him to know he can do more than what people expect him to be.
The characters must FIGHT for each other-
Alex and Brittany from Simone Elkles's Perfect Chemistry because of how much they are willing to do for each other. No matter what happens, they find a way to make things work. I hate when a love interest says they can't be together in order to protect the other. I call bull. If the love is worth it, they fight for it no matter the circumstances. You don't give up on love.
The characters must UNDERSTAND each other-
I love Hazel and Augustus in John Green's The Fault in our Stars because of how the understand each other. They both have cancer which bonds them together. They both understand what the other is going through because they went through it too.
The characters must BE THERE for each other-
I love Veronica and Logan in Veronica Mars because they are epic. Sorry had to say that. I don't know what I love most about them. There is just something so magical about their relationship. I guess I love how they are there for each other. In the beginning of the second season, we see Veronica comforting Logan. At the end of the second season, we see Logan comforting Veronica. I love the cycle of it. In both scenes, they weren't even "together" (I put air quotes because I fully believe that Veronica and Logan are always together forever. They re that epic), but they are still there. No matter what happens in their relationship, you KNOW that they other will face hell to be there for the other.
Other notable relationships:
Anna and Etienne in Anna and the French Kiss by Stephanie Perkins
Carlos and Kiera in Rules of Attraction by Simone Elkles
Emme and Ethan in Take a Bow by Elizabeth Eulberg
Courtney and Jordan in Two-Way Street by Lauren Barnholdt
Buffy and Angel in Buffy the Vampire Slayer (I love Spike too, but...)