It’s probably true that they don’t know why they hate each other. The first four lines of the play come from the prologue and don’t specify a reason.
Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
Notice that Shakespeare says here that their grudge (fight) is ancient. It’s been going on a long time, but he doesn’t say why they have a grudge. Shakespeare is content to just say that there is a feud without explaining it.
It’s also true that the characters seem to have fun with the feud. Mercutio, Tybalt, Samson, and Gregory seem to enjoy the back and forth, until people start ending up dead. I think that Shakespeare, rather than considering it a “joke,” is looking at how passionate and unthinking young people can be. They do what they are excited about doing, and to heck with the consequences. This leads to deaths of multiple characters, including Romeo and Juliet.