One of my favourite reactions from Kaidan arrive in Mass Effect 3. It appears pretty early on and I think it depends on Shepard having been in a relationship with Kaidan previously. I totally do, almost every playthrough (except one memorable Renegade session where I played male Shepard and broke all the rules. Liara still loved me.), and I probably will in the future as well. Any other option seems wasteful to me. Besides, Ashley is a space racist.
This moment occurs when Shepard and Kaidan discuss Shepard’s stint siding – sort of – with Cerberus. They discover extensive changes that the Illusive Man has made to his troops and Kaidan asks if Cerberus did this to Shepard. When Shepard responds with “hell, you were always stubborn”, this little piece just… got me. It hit me like a freight train, and I have no idea why.