It took Leon a long time to begin trusting you, at first he was hesitant about the entire relationship lasting, but after some time he began to hope and think that maybe this one would turn out okay.
He slowly began opening up to you, revealing things and being more open, he'd tell you his secrets and thoughts, not just basic ones but deeply emotional things he'd never share with anyone else.
Leon started noticing how he'd think about you all the time, everything would come back to you, and you became his reason for surivival alongside his need to protect and save others.
The first time he said "I love you" was in desperation. You both were in a sticky situation and had no obvious way out, Leon didn't want to die without telling you how he felt, so when he thought he was going to die he blurted it out. He held eye contact and gave you a look alongside the words to express everything else, but those 3 words meant everything to you.
Afterwards when the world hadn't ended and you both managed to escape, Leon felt kind of embarrassed, but also relieved he'd finally told you. Covered in blood and other things, he stalked up to you and grabbed your face, kissing you passionately whilst repeating "I love you so fucking much."
He knew from then on there'd be nobody else but you.