The scene that you are referring to is Act Three, scene one. In it Benvolio and Mercutio are talking to each other. Not sure about what, because when the scene starts, Benvolio immediately states that they should knock off for the day and rest. His opening four lines say exactly why he believes that tempers will be quick.
"I pray thee, good Mercutio, let’s retire.
The day is hot; the Capulets, abroad;
And if we meet we shall not ’scape a brawl,
For now, these hot days, is the mad blood stirring."
To paraphrase, Benvolio is asking that he and Mercutio go inside because it is a hot day. People tend to be cranky and irritable on hot days.
Benvolio also knows that the Capulets are likely to be about. Life-long sworn enemies of Benvolio and Mercutio out wandering around on a hot day with irritable and cranky attitudes does not bode well for Benvolio. It's not stated that Mercutio has a hot temper, in fact Mercutio accuses Benvolio of being quick tempered, but it's obvious through Mercutio's actions throughout the play that he is the quick tempered one. I'm sure that Benvolio knows this and is looking to get Mercutio out of there before any violence starts.
No such luck since by the end of the scene Mercutio and Tybalt are both dead.