During a past mission, Simon had sacrificed himself to be able to save you. The Russian soldiers caught him instead of you and took him away. That day was a nightmare, lots had lost their lives, some were injured, yet you were fine. You hated it. It wasn't fair. Why didn't they take you instead of Simon?
Simon had dreams.
Not you. You don't dream, you just live day by day. Depressed. You had always been a lost cause. That's what you thought before Simon helped you change your perspective in life. He had helped you through a lot of your troubles. Like Anxiety, s-h addiction and your e-d. He was so gentle with you despite him being a scary Lieutenant. You got better, and chose him as your reason to stay alive. You lived for him, and if he wasn't here anymore... You wouldn't find another reason to stay alive.
3 days had passed since Simon's disappearance, after numerous researches and failed localizations, he was classified MIA. Capt. Price gave the order to change it onto KIA after 5 more days if they couldn't relocate him. Price knew about your relationship with Simon and how out of it you were, he felt devastated. But he had to make sure you wouldn't do anything reckless, so he warned you.
'Do not go try and find him, we'll lose you too.' Was what he said to you that evening.
It was now night, the silence was loud in the now empty hallways of the base. Everyone was already sleeping in their rooms. Yet you were laying in bed staring at the open window, the moonlight caressing the dark room. The once warm bed with Simon holding you to sleep was now empty and cold as ice. You had relapsed and stopped eating since you came back from that mission.
It wasn't fair. It should've been you who got caught, not Simon.
The frustration, guilt and regret filled your empty self. That's when you made your decision.
You will find Simon. You'll sneak out and go look for him on the site where you have last seen him.
You didn't care if you would die. You were already dead mentally, without him.