I don't know for sure but I think it's because the pictures show him in love with someone else while Taku photographed him so lovingly. Like he's looking away in the photo and looks vulnerable and seeing how obvious it looks in the picture might have made him cry. But this is 100% speculation.
Basically the concept for that photoshoot was something like ‘imagine you’re in a room with the person you love and you suddenly can’t see them anymore’ haesoo thought of joowon at that point and he cried in the photo, in this chapter he remembered that and cried again, I believe in the next chapter he also says something like ‘it’s felt like something in my life had been cut off/out and I don’t want to leave it that way’
Doe anyone knows why haesoo was crying?Was it because of the picture or something else... some please spoil it for me （￣へ￣）