I depict Kris with Frisk like Sora with Roxas. Anything Kris sees that made Frisk so happy would tug his SOUL greatly making him shed a tear. Frisk had to say goodbye to Asriel so seeing someone that reminds them of him would get them very emotional. Not to mention probably just seeing older Asriel after college would really hurt him. It's so happy, but at the same time it's all a lie.
I depict Kris with Frisk like Sora with Roxas. Anything Kris sees that made Frisk so happy would tug