The first time Castiel realized he would die for his father was the day he was created, however many millennia ago. It was what he was created for; it was his purpose. He was merely a puppet on the strings of his father, the great puppeteer's hands. He was created with the knowledge that if his father wanted him to die, he would die. He was brought into existence knowing that eventually he would die for his father. And he was okay with that.
The first time Castiel realized he would die for a human was the day he fought like hell and through hell to save him. It was the time he saw Dean Winchester who died to save his brother and knew that he deserved to be saved, deserved to be died for. It was when he saw green eyes and called them home.
The first time Castiel realized he would die for himself was when he was at his darkest moment. The Winchester had disappeared; his father had abandoned the earth once again, He felt hopeless, and he wanted out.
This point was the first time Castiel realized that not only would he die for a human, he would live for one too. He would find the Winchesters, he would move on without God, and he would survive.
The first time Castiel, against all his better judgment, realized he would die for a demon was when he met her. He admired her strength, he was intrigued by her smile, and he loved her determination. Here was someone who was as hopeful about and devoted to a cause as he was. Here was someone who was willing to die for what they believed in, just like he was. When he met Meg Masters, he didn't see what she was, but rather who she was. When he looked at her, he didn't see her soul, but her heart.
The first time Castiel discovered he would die for humanity, he did - at the hands of his brother. He died so that Dean Winchester could save the world. In that moment he could never know how much he would shape humanity, or how much that moment would shape him.
The first time Castiel realized a human would die for him, changed his life. He was the one to make the sacrifice. He had always been. They were in Purgatory, and Dean had fought through monsters and terrain to find him. It had never clicked for Castiel that someone would die for him. That was his job.
The first time Castiel realized a demon would die for him, was after it had already happened. Crowley had told him when they were searching for Lucifer. Meg Masters was dead. She had died to save Castiel and his Winchesters. Sometimes Cas wondered if he was doing the right thing by letting her live. To an angel, demons were an abomination. But in that moment he knew he had done the right thing because Meg Masters had done the right thing.
The last time Castiel realized he would die for the Winchesters, was right before he did. He had seen too many people die for him, and he refused to let it happen anymore. If his death could, just for a moment, help them. If it angered them enough to do what needed be done; if he could protect them, he would. And he did.