It has been reported that Song Joong Ki and Song Hye Kyo met each other even before Descendants of the Sun!
The July 10 broadcast of Channel As Heard It Through the Grapevine discussed the two actors upcoming marriage.
During the show, one of the panelists revealed that the couple first met each other in 2013 through Jo In Sung, who had starred in the drama That Winter, the Wind Blows with Song Hye Kyo.
Another panelist then added, Many actors, such as Jo In Sung, Kim Woo Bin, Lee Min Ho, and Kang Dong Won, were in talks for the role of Yoo Si Jin in Descendants of the Sun. However, most of them had to turn it down because the drama was being pre-produced and filmed overseas. Jo In Sung then recommended Song Joong Ki to the production staff.
It looks like Jo In Sung played a pretty significant role in this whole thing! Meanwhile, you can read more of Song Joong Ki and Song Hye Kyos love story here.