There's someone analyse that SW is the one change most after 6 years skipped. Not only his hairstyle but also how he looks. More mature and more neglect.
Before except the time he was late for school and didnt want BR to wait in the cold, SW always walked slowly and looked around. He was the last one came to JH's house and also the last one came out from JH's room in ep2. Whenever he crossed by the Sung-Kim's house, he always stared into their yard to check if BR was there. He never rushed to walk home after studying in the midnight.
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But after the time jump, SW walked very fast with his eyes looked down. He passed the front stair and never paid a look to it. That stairs used to be the place he patiently stood waiting for BR under the snow, in the cold of winter and during the midnight... And now he even didnt dare to look at it. But when he saw BR, he just bowed. It was even colder than ignoring her. A bow means 3 month-dating in 89 and all the crush for her no longer exist TT_TT
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All of the scenes were described very detail and carefully by the actors and PDs. And the 94 scene was really heart-breaking when you link it to the scene he held a purple pillow when sleeping or when he told Taek he had to use medicine to sleep... 6 years was really tough.
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There's also a fanfic explaining why Noeul said SW was good at dating. A friend summarized it to me and it really heart-breaking just to hear the brief translation.
The fanfic tells that during those 6 yrs, SW once was forced to join a blind date with a group of friends. He acted cold and didnt speak anything or look at anyone during the meeting. Until everyone was about to leave, he noticed the girl pairing with him had a mole under her eyelid and finally he smiled (~you know--- "even the mole under her eyelid is beautiful") That girl, of course, didnt ignore a perfect guy like SW (doctor, scholarship student, graceful son, nice and polite guy....) so she followed him. She even visited his house and his mother.... SW treated her nice too (well, he's nice with everyone) and thought she was OK to be a ideal gf (pretty, solf manners, sweet voice,...) but there was nothing like the way he did to BR, he never waited for her, never rushed to meet her, and during his busy schedule in hospital, he totally forgot to call her... One day SW came home and that girl was there, he walked her to the bus stop. The girl tried to hold his hand and he realized her hand was so warm (gosh~ warm hand~) And right then, the bus arrived and Noeul came down :))) So that No Eul saw it all and reported to his sister :)))