This contains possible spoilers, although it's simply a theory.
enjoy.
PROLOGUE
"I can't let you go out running through the streets declaring war without any backup! It's too dangerous!".
Castle had dropped by when Beckett was kicked off the case and on forced leave, they had been arguing.
He knew her well. As of late, she had been on her own running rogue and with Castle following close behind.
Since Lockwood's grand escape, and two officers getting shot and killed it became all too real again.
She had to push everybody away, Ryan and Esposito included. There would've been a whole bevy of laws broken, had she kept them both on her side of the war.
"This was never your fight, Castle!"
"I'm scared, Kate. I'm scared that you're going to get yourself killed pursuing this. We can get those people responsible, just don't do it alone...".
As his voice dropped with the last 5 words that left his lips, it became obvious that those words meant more out of context.
He turned his head away from her, hiding the well of tears filling his eyes.
"Why do you care so much about how I pursue my mother's killer?".
"Because I CARE!... I..."
"WHY...Castle?"
He locked his eyes back into hers, finally facing the truth that has kept him around for the past 2.5 years.
"...I care about you. More than I probably should."
Finally... Confirmation that hit her like a freight train at full speed. In the back of her mind.. She knew, but she had brushed it off.
She looked at the man standing there, standing like he was defeated. He had just admitted to her that he was in love with her.
She stepped closer to him until mere inches separated them both, she took his hand into her own.
He still has yet to acknowledge her, his proclamation has rendered him vulnerable as to what may come next.
"Castle... Look at me."
Her voice became soft, unlike a few moments earlier when he came storming into her loft. He raised his head, and their eyes met.
"It's too dangerous. I don't want you to get hurt, you're not trained in any way...I can't ask you to take a life on my behalf.
I can't let you do this to Alexis, to your mother. You mean too much to me for me to ask to to put yourself in that position."
"Bull. You know I'm just as good of a shot as you are."
The stark sudden silence surfaced again, and this time she knew she had to speak the truth.
"I can't let you...because..."
"Because what, Kate?"
He pushed her, he wanted to hear it.
She heard those words again in her head, those words that Royce had written in the letter to her. 'Putting the job ahead of your heart is a mistake...'.
She wanted so bad to heed those words, but this was a special circumstance.
She knew it was dangerous to let her heart rule, not when she was this close.
"Kate?"
"I need to be alone right now."
She was shutting herself off to him again, just like the time when they were in LA, when she felt her walls crumbling down.
Walking over to her door, she opened it and waited for Castle to leave.
She felt his eyes on him, his confusion, his fear.
He left silently, leaving her to figure things out before she did anything stupid.
This was the risk that she was willing to take, and with the close of the door it felt like she had just lost him then and there.
Her body felt weak from the deluge of emotions flooding her body, he leaned against the door for support.
She couldn't hold back her emotions any longer, as her walls were eroded by the man she just asked to leave.
He stood out on the other side of the door, hearing her cries through the door.
It pained him to experience this. The mystery that was this woman, had captured his heart and he couldn't be there for her.
He turns to leave, but not before he hears her voice whispering
"...Because I love you."