Why did Mr. J and Harley jump into the chemical bath in Suicide Squad?
My interpretation is that this was the moment J knew he loved Harley, whereas before it would have been Harley that had to "pass every test" to prove her love for J, this scene shows us that now J is being tested by Harley to prove his love for her.
J: Would you die for me?
J: That's too easy, would you live for me?
Harley, then J, dive into a chemical bath
EDIT: Possible duplicate. I did try and find it before asking, maybe not hard enough, I think it is because I couldn't find a better search query for "chemical bath", hence the lengthy title of my question to prevent future duplications: