Benvolio suggests this because he is in fear of bloodshed. the first spoken piece is from belvolio saying " I pray thee, good Mercutio, let's retire the day is hot, the capulets abroad, if we meet we shall not scrape the brawl for now these hot days in Mad blood stirring." he is also saying that the Capulets have gon mad and something is a brew. it is easier to avoid then get caught in the crossfire.
I hope this helps in any way. my shakspearian translations are a little rusty.
