I found this article on Soompi .. and thanks to Sis Myra too!
Through an interview with Sports Chosun, Jang Geun Suk revealed that he had suffered from depression due to his excessive workload a while ago.
After filming for drama “Mary Stayed Out All Night”, he toured around Asia for fan meetings, and began filming for movie “You’re my Pet”. Not to mention he released singles and full-length album in between all these works; he even squeezed out time to film commercials. “In total, I had less than 25 days to rest in the past year.”
He further revealed that he had suffered from depression a while ago, “Fans tend to think I was taking a rest when I was not on TV, yet I was actually traveling abroad for work. Whether it was in Japan, Taiwan, or Singapore, I felt like I didn’t belong to anywhere.”
He added, “The busier I am, the more distant I become from my friends. I love cars, but all I could do was leave my car in the parking lot and let my manager drive me all the time; I felt like I was living someone else’s life which made me really depressed.” The star was so depressed that he didn’t step out of his house for two months.
Fortunately he conquered it on his own in the end, “It eventually got better when I was filming for the movie “You’re my Pet”. The filming schedule was so regular that I slept really well, got up at 10am and went to bed at 1am everyday. I went to Taiwan for fan meeting during the filming, it was when I stood on the stage and heard cheers from fans made me realize, this is where I belong, and this is what makes me happy.”
I know this should be up on the JKS thread ... but what I'll say is something related to GEUN♥SHIN...
This is an effect of being away from Shin Hye...like we said before...it's withdrawal syndrome...separation anxiety.

Now I know the reason why SH tweet before sounded lonely and anxious too...and this was why they did not go out for walks too often...she had to stay home to take care of him... and if you noticed,
SUKKIE MENTIONED THAT HE ONLY REALIZED IT WHEN HE WENT TO TAIWAN..
.he specifically mentioned Taiwan...because it reminded him of her...this is where they had good times...even said this is where I belong...We all know Sukkie...he may mean two different things here. REMEMBER IT IS IN TAIWAN WHERE HE CHOSE GMN INSTEAD OF THE OTHER CHARACTER IN THE OTHER DRAMA....WHERE HE SAID "AM I TOO OBVIOUS NOW?" when the MC mentioned that he smiled when GMN was mentioned... Shin Hye is his hiding place...she is his haven...she is the only one who can make him feel better. And working too hard made them not see each other often..even until now.
I'm happy to know that he is fine now...so that is the reason why the smile faded away and then came back. I still remember he looked worse in Thailand....the time of the Baeksang...so this was why... but I know he is happier now...until SH accident a couple of days ago...
He is missing his old self...the old Keun Suk...when he was still Hwang Tae Kyung...when he was with his Go Mi Nam...when he was working with Park Shin Hye. And he knows he can only get that back when he gets to do a project with Shin Hye again...and this is why I think that he will...that he is working on it. I even think that he talked to the HONG SISTERS to make them do a sequel for YAB because this is the only way he can work with Shin Hye again.
I pray that both of them will be better soon.
To Sukkie and Shin Hye...we love you. Always remember that we will be here for you even if those 'so-called fans' of yours Sukkie abandon you and leave you because you want to be true to yourself.
Guys, let us hope and pray that they will be okay soon. Sukkie is slowly getting back to his old self. But pray harder.
Geun♥Shin hwaiting!