I don’t think I can explain why ANYTHING is vexing: I think it’s always a non-optimal response.
But I’ve learned from reading and talking to people and watching TV and film fiction, that watching teanagers make out is widely held to be vexing, and it doesn’t get any less vexing if the individuals are older.
PS: the OP didn’t go the extra mile and call the bus driver ‘an ass’. The original question asked about his actions, not his personality.