Well, I've been completely head-over-heels for him for ages, so I would be ecstatic. However, he thinks of me like a sister and has already rejected me because of that, so the chance of him ever saying something like this is incredibly low. In addition to the immense joy, I would also slightly shocked. I mean, he's made it perfectly clear that I'm like a sister to him, so he would never say something like this to me. If he really meant it, then I would tell him I love him too, throw my arms around him, and finally kiss him.