Responses

Only theories say bcz he looked miserable and depressed in that pic .. remember the concept of the pic? Was to imagine yourself having sex with someone you love but you can't be with him anymore and haesoo ofc thought about joowon .. so when haesoo saw the picture he probably realized how much grief he felt after breaking with joowon and subconsciously teared up... we do believe he is not in the place he actually wants
In Taku's exhibit when haesoo was staring at Taku he sheds a tear. Idk why but it was korean but that's what happened. He literally sheds a tear when he was looking at Taku.