And here's the thing Kate's learned about their love. Yes, it's real and it's right and it's imperative. No, it isn't easy and yes they'll always bicker and fight. He'll want to stand by her side even if it meant death and she'll always want to run head first into danger if it meant keeping him safe. Their love is anything but selfless, though people looking who didn't know them would disagree. Of course, they also don't know the story behind their love and how they've kept it alive. And maybe Kate can't live without him. She doesn't want to try and find out regardless. And he knows life without her is impossible. They're selfless to those they love, but when it comes to each other they are possessive and headstrong.
Kate and Rick compliment each other well it's the truth. It's not something so silly that they complete each other, but they do make each other better. What one lacks, the other possesses. He knows how to bring her back from the darkness and she keeps him grounded in reality. The give and take they portray in their daily banter is reflected down to their very souls; it's how they stitched themselves together.
It's how their relationship formed, through the many trials and tribulations they've conquered together. Beginning when he first saved her life when they were hunting down fake purses and culminating into reality when he jumped Lockwood and punched him out so Kate could live to see another day. They faced down a freezing death wrapped in each other's arms. All they needed to do was hold hands when being blown up was an option - their love had never truly needed verbalization to be known and understood, though the sound of it on each other's lips was reason enough to say it - their eyes saying everything their mouths could never form.
But now, as Kate lay bleeding on the grass in the cemetery, she realized the one truth about her love for Richard Castle, the one that underscored the rest of it. Their love gave her a reason to live. When all else failed, her strength, her will, her hope, when her mother's case blew up in her face and betrayal and lies met her at every side, Rick was there. Steady as a rock. Arms out to catch her and hold her. Fingers laced with hers, his gentle pulse a reminder that life was good and worth living. Understanding etched in every feature, love shining in his blue eyes.
And though it's never been needed before, Kate understands as her body pulls her under why he tells her the obvious. It's not a declaration she doesn't know; it's a reason for her to come back. To stay with him. Because he loves her.