The line of Detective Roger Murtaugh(played by the wonderful Danny Glover)has been canonized as one of the best lines in cinema history and for a generation. Much is the same with the Bad Boys movies. Although the 1995 movie has produced one sequel and another hopefully in the works, it has become for a generation what the Lethal Weapon series of the late eighties and early nineties did: a go-to buddy cop, action-comedy.
The plot is even similar
On one side you have the bachelor detective (Gibson and Smith) and on the other side there is the married detective with a family (Glover and Lawrence) and everything to lose, as compared to relatively nothing to lose for the bachelor detective.
Even though the characters played by Smith and Gibson are not related to their counterparts in Lawrence and Danny Glover, they seem to be a part of the family at any rate.
One of the things lacking in the Bad Boys movie series is a Special Forces past. In Lethal Weapon, Martin Riggs (played by Mel Gibson) was a retired Special Forces soldier who had served in the Vietnam War under the highly secretive Special Observations Group that conducted reconnaissance missions over the border in Laos and Cambodia. Will Smith, who plays Mike Lowrey, is not military-grade but rather the stereotypical bachelor in Miami. He sleeps with women and is not nailed down to any of them (at least in the first installment).
This leads to another point.
While maybe too obvious, there is the romantic side of things, where the bachelor meets the girl he wants to settle down with. For Lethal Weapon, Martin Riggs found that in Rene Russo's character, Lorna Cole. Cole was the Internal Affairs sergeant and the female version of Riggs. What we find in the Bad Boys movies is essentially the same thing.
Lowrey falls for Syd Burnett, Martin Lawrence's character's sister. Syd is played by the gorgeous Gabrielle Union and is very much like Mike Lowrey. She is even a fellow police officer.
Both movie series' have fantastic dialogue between the two and the thrills spill into each respective sequel. With the similarities we see, it's only fair to have Bad Boys make four total movies.