Monkey D. Luffy had always been a shit-storm since the beginning, and frankly no one could deny that. Especially when it came to Trafalgar D. Water Law. He first read about the carefree 17-year old from the news; he had beat up Iron-Mace Alvida, caused a ruckus at a Marine Base, beat up Don Kreig, and ruthlessly defeated Kuro of a Hundred Plans & Arlong, (All of which took place somewhere in the East Blue area.) which gave him his first bounty 30,000,000. He brushed this off at first, but then not even a few months later, more news came in about the rising Straw Hat crew.

The Straw-Hat Pirates, (as the Marines labelled them) their captain had defeated one of the seven Warlords, Crocodile. They'd declared war on the damn world, at Enies Lobby as they burned the World Government flag, all to save one crew member. Then came the Sabaody incident. Luffy, fucking punched a noble. No one heard from him up until the announcement that Portgas D. Ace was going to be executed for all to see, at Marineford, which evidently caused the Whitebeard Pirates to appear. Who he didn't expect to appear was Monkey D. Luffy. (and without his crew, also!) (and also, Law thought the Marines were incredibly stupid for basically declaring war of one of the four Yonkō)

Although he only interacted with the boy once, at Sabaody against the Marines, he knew something was going to happen to him Luffy, and how right he was. Monkey D. Luffy's origins were explained- he grew up with his grandfather, Monkey D. Garp, whom of which was the father of Monkey D. Dragon. Then Sengoku stated that Luffy and Ace grew up together, and eventually the two, Mugiwara no Luffy and Hiken no Ace, were step-siblings. ("what the hell?" Law thought to himself) Granted, it'd be a shame if the boy died before his "time,' wouldn't it? So that's why he decided to go directly to Marineford. (Much to his crew's disbelief.

He saved Luffy's life and escaped, (although barely) and although he couldn't help but feel bad for Luffy for losing one of his loved ones (just as he had lost Rocinante) he knew that this would be a fun operation.

He's rather surprised the Pirate Empress allowed Luffy onto her island. (but then again he always had a way of charming people...) Eventually the teen recovered, only to have common sense knocked into him by Rayleigh (when the hell had Luffy become chummy with the first mate of the Pirate King anyway?)

He meets Luffy once more at Punk Hazard. The over-energetic kid thanks him for the help during the Marineford incident, and Law shrugs it off, only giving a simple "You're Welcome," and leaves it like that.

Later on, he realizes, he can use Luffy for his goals. And so, he brings up the alliance proposition, to which Luffy enthusiastically responds with a large "OKAY!" After bringing the Straw-Hat Pirates realized he agreed to this, they freak out. (But Law's used to people being scared of him, it was normal response. Especially, after he caught Flevance's White Lead Syndrome.) The only calm one is Robin, who goes along with Luffy & Law's alliance, but brings up how betrayal is quite common in these situations. He turns to Law and asks him if he'll betray him. Law replies will a dull "no." The boy turns to his crew (well, more like half of them but, hey-) with stars in his eyes and an unwavering expression. He compliments them and his crew falters, and decides to go along with Luffy's plan.

Usopp warns Law of Luffy's idea of an alliance, and to be frank he probably should've called it off at that point, but oh well, it was too late for that, at that point. Because, Law looks back on that time, and realizes that as soon as they both agreed to the alliance, Luffy thought of him as a friend and wouldn't let him leave. He finds himself impressed by Luffy's unshakable will to take down Caesar Clown. But then again, Caesar did do terrible things during his time as a scientist, and obviously Luffy wouldn't let him live it down. (no way, especially since Caesar tried to kill his nakama with poison gas.)

As he travels with the Straw-Hats, Law finds himself shocked, especially with Luffy's nicknames specifically made for him...

"Torao" was one of the many... he pretended he hated them, but honestly he felt a little warm inside every time he heard the 19 year old boy call out his names on the deck of the Sunny.

But, in the end, he finds himself indulging in their antics, and Law's alright with it.

After the Dressrosa incident (which was fairly recent, actually) he comes to the the conclusion that Luffy was a crazy son of a gun and that his nakama meant everything to him, and that he would sacrifice himself for them; he would defeat anyone who dared hurt them. He knew this whole-heartedly, because, he was one of them. And for the first time, (in a long time, actually) he feels loved and accepted by people (except for his crew, that is. He knew they cared about him) for who and what he is. And he's happy, and let's out a genuine smile, as he reminisces on the times he learned about Luffy, and met Luffy. (or Mugiwara-ya as he tended to call him)