Descendant of the Sun Recap Episode 8
Shi-Jin mentions no matter how hard he drank, tried to tire himself out, he kept on recalling her. He asked her if this was something she never thought about and he told her to think about it, because he is serious this time.
The meaning is possibly he wasn't serious prior and now he is in too deep. Totally different person compare to the happy go lucky flirt persona that he display to her prior hand. I suppose whilst in Korea and trying to get over her he couldn't forget her and ended up scared when he noted that she was in a disaster area.
