The Rules.
They all knew them.
Most of them were rather obvious. Easy to follow.
For example, Rule 2: "Always wear gloves at a crime scene." It's part of basic agent training.
Some were also a given. Probably not glaringly obvious, but they make complete sense. Usually unspoken, anywhere else, but Gibbs' Rules give people that extra push - incase they forget the basics sometimes.
Rule 1: "Never let suspects stay together." and the other Rule 1: "Never screw over your partner."
The latter was spoken as a reminder more often than one would expect.
And the two Rule 3's - "Don't believe what you're told. Double check." and "Never be unreachable."
The latter was broken quite often. Sometimes it was out of people's control - being kidnapped was a common occurrence with the team, and kidnappers know not to let their victims use their phones. The smart ones remove the batteries. Or abandon them somewhere.
The team went through many phones because of kidnapping, and other similar circumstances. The team didn't always have the best of luck.
Then there's the commonly broken rules - Rule 6: "Never say you're sorry. It's a sign of weakness." and Rule 10: "Never get personally involved in a case."
Gibbs broke the latter most often of the group.
Rule 22: "Never ever bother Gibbs in interrogation." - now sometime they can't help but break this rule - if Gibbs was in interrogation, and new evidence, or results of tests, came back, and it pointed away from the suspect he was interrogating, then they couldn't let him carry on without all the facts.
And sometimes they had facts that would give them the upper hand. Evidence that would finally lead to a confession from the suspect.
New rules, that come into place later, started with Rule 51: "Sometimes you're wrong."
Which lead to more rules being made, after the original 50.
Like Rule 69: "Never trust a woman who doesn't trust her man."
Rule 12 was one of the hardest to follow, "Never date a co-worker." Especially when you feel like you should follow your heart.
Gibbs and Jenny. McGee and Abby (though that had started before they began working together). Tony and Kate. Tony and Ziva.
Tony really was wayward with the rules.
Rule 12 does get followed though, and the three men - and Abby too - had found happiness with other people. It may not last long, but they've all found other people to date. And possibly love.
Sometimes it might hurt, when they think about their lost loves - death, moving away, or it just not working out - but they get over it. Smother the feelings. Sometimes that can look back on it fondly. But when it hurts, it hurts bad.
Maybe that's why Rule 12 existed. To save from hurt. It didn't help though. You can't control your heart.