Hello sisters. Since sister Judy started an interesting topic , I will share what I feel with you.
I do not believe Jang keun Suk and Park Shin Hye were a couple since years ago. Let me explain , I believe Sukkie sees PSH as the one he wants to marry , he is attracted to her in every way possible and through the years , no matter how much time has passed he still feels deep love towards her.
PSH on the other hand , she cares about him deeply and is attracted to him and I believe deep inside she loves him , but she's afraid. She has always put a limit to his advances IMO and wanted to take things slowly since they were so young. I think JKS agreed with her silently , they never talked openly about their feelings not even with each other , but both know it deep inside. They feel the blinding chemistry and they miss each other terribly. That's why they kept doing things together. That's why JKS said in his world cup variety show that he might date but he wants to marry PSH. JKS made sure PSH knows he wants her as his bride and I think he's the one who even encouraged his mother to say it at the top of her lungs. but I think PSH purposely took it as JKS being playful and throwing jokes because she knows deep down they are still too young to be ready and they are also endangering their friendship encase things didn't work out. She wasn't ready ( and she have every right not to be ) sometimes beautiful friendships gets ruined when the two parties decide to take their friendship into the next level.
I believe JKS told the truth when he said he didn't date for the last three years. He just went about his life and acted as the best friend and brother PSH needs. Then comes the famous show " Knee drop Guru" and JKS admits he's interested in one of the girls he's rumored with but he never told her directly , Take note , after the show aired JKS immediately tweets PSH and tells her ( did you watch the show? Did you heart flutter?) That can only mean one thing , JKS was telling her " I meant you when I said I am interested in one of them" but still he didn't get a sign from SH that tells him "the light is green , you can come out and confess" So he kept the best friend - brother thing going.
JKS's friends started getting married , he tweets about how they are growing old and how time is running without them feeling it. He even says he's really jealous. ( I think three or four of his friends got married in the last four months) JKS started to feel even more lonely than usual and Love Rain , a love story that goes through generations has been his escape. I think he got too much into the role and Hana's character ( not Yoona herself but Hana the character ) is the closest to PSH's real personality. He fell completely to the role and that's why I think , just before the Beaksang awards he tried to change things between him and Shin Hye but she didn't agree with him. I think it was something close to rejection but wasn't exactly that ( I don't know if I make sense to you but it makes perfect sense to me

) .That's why they were both awkward in the ceremony. Then Shin Hye tweets about " Beautiful memories being ruined and that she's so sad" .
After that day JKS was a real mess , he started loosing sleep , always having bags under his eyes. His smile was barely there and if it was it didn't reach his eyes. He kept tweeting he feels lonely and even once got on Live TV looking completely like a mess. Barely awake and with no make up or hair treatment and then he said this line in that " I am good in a lot of things but not in Love "
I think both got time to think in this period and started to get back on track again , we got the Zikzin band tweet ( I think it was a very sweet move from Shin Hye , she purposely tweeted five minutes after he tweeted wearing the same hair band that was designed by him) Also we got PSH tweeting about her mother and JKS took the chance to again remind her he still think of her as the girl he wants to marry so we got the famous "mother in law" tweet. Then , something happened between eels and JKS. JKS was hurt badly because of it and got terribly sick after it. He lost weight fast and his staff kept tweeting about how worried they were. He was seen a couple of times having aid to breath after concerts. I think at that hard time PSH ( through talking and texting) helped him a lot and in that time I think she finally crossed the line of friendship and allowed things to go beyond friendship. JKS met with children , talked about having children and then took a sudden leave from everything including work , twitter and Apps. And now he comes back with his bright smile! We finally see him as happy as he was before. I think ( or hope) PSH has a lot to do with it

I was so happy with SH tweet about the food in colamercato and how she was talking to Sukkie. They are still as close as ever and that's enough to make me happy

And now , SH suddenly announced an Asia tour in celebration of he 10th anniversary.! Another clue to their influence on each other. They influence each other as if they are life partners!
That's only my opinion. It could be totally wrong. It's just my look on things

I would like to hear what each of you think. I enjoyed reading sis Judy's opinion. Hope everyone can post what they think.