I think to put this simply, as the weather increased, tensions grew thicker. Which spurred along action from our characters, thus revealing some of their more unsavory traits. You know, since heat is uncomfortable and stuff.
Jay is in complete denial about daisy being in love with tom, which basically shatters him as a person, because he can't get what he wants.
Which is kind of awkward because it's revealed that daisy sort of treats their affair as a bit of a joke, because the only reason she kisses Gatsby is to get back at tom.
both of these events reveals a "side" of these characters that hasn't been shown before
Myrtle also straight up dies. Ran over by Gatsby's yellow convertible Gusenberg, which, if you looked up 'how to clout", you'd probably find a picture of that car.
And Nick walks away from it all, deciding that he's "had enough" of the Buchanan's.