Jason never thought he was the type for domesticity. He always led a life of excitement, and danger, and death (quite literally). Between his childhood on the street, his stint as Robin, his death, his revenge, and his vigilante work, he could never catch a break.
And he got used to the constant pain. Until you came along. You, who broke down all his defenses. Who taught him it was okay to be vulnerable sometimes.
He knew he loved you. He just knew. So, on your 5th anniversary, he brought you to a nice restaurant and tried (and failed) to work up the courage to pop the question.
The velvet box in his pocket felt like it was burning a hole through his slacks, and it was all he could do not to pass out, watching you eat your steak dinner without a care in the world. This was it. He had to say something. But every time he tried, his words would get caught in his throat and he turned an embarrassing shade of red, halting his efforts.