How come U.S-South Korea relations are so good?

I'm a korean-american guy and I just want to understand why and how the U.S. and South Korea became allies and flourishing partners. I'm not saying I'm against it, I'm for it (proud to be american and proud of my ethnic heritage). U.S-Japan relations are also extremely good and China its a bit of a mixed bag obviously. So, the U.S. has a enormous influence in South Korea and South Korea has a lesser influence in the States but its a solid influence and rising, I believe. Was it because of the Korean War? U.S. helping SK against NK and China? What are our relations?