He didn't regret it one moment, to have sacrificed his arm back then.
Back then, when he saved Luffy's life while he chased the Sea King away with his Haki. There would be never a doubt if he did the right thing at that moment.

For Shanks, it was clear as day, the memory of this moment. Luffy's body pressed to his, the hot tears on his cheeks, his own pain in his shoulder. But he managed in seconds to subdue the pain and ignored it. Because the pride, which filled his chest, was way stronger as any possible earthly pain could be.
Yes, he was extremely proud of Luffy, at this moment he chased away the Monster. The red-haired Yonko still didn't know why, for sure.

Maybe, because Makino told him why Luffy enraged the bandits. Or maybe, because Luffy was so brave.
Or perhaps it was just because Shanks knew, with his deed, he changed something inside this little boy and probably because, without him wanting it, Luffy became something like his biggest pride. Wouldn't he know better, he would have called the feelings he felt and actually still feels paternal instincts.

But whatever it was, which made Shanks going at this moment and made him so proud at that little boy who couldn't even swim and would never be able to learn it, because of the devil's fruit he ate, it was without a doubt the only right thing to do at that situation. He remembered precisely how Luffy yelled at him with a desperate voice, when he realized that the Sea King chocked off his arm. His small fists beat steadily on his chest. He clutched on him and the Redhead could see the pain, the guilt and the fear in the eyes of the small one. His words, he would repeat them, now as he did back then. They were still the only truth to him.
"Come on, boys don't cry. It's nothing, it's just an arm. As long as you're alive …"

Yes, as long as Luffy was alive – and he was sure the rubber boy would survive him – Shanks would be always proud of him. By far this scar was the one he treasured the most. Nothing like the three scars over his left eye, even though they didn't affect him as his lost arm did.
It was strange, the three scars hurt him more although they should inflict less pain. But it was this thing with scars, they carried memories and if the memories were good it would cover the pain. Was the memory painful, the scar was as well. But if the scar triggered hate, than no memory of the world could cover it. At least it was his explanation for all this scar-thing.

Back then, when he sacrificed his arm, he got used to it pretty fast. Of course it was a readjustment.
He always had his two arms and a lot of things were new with only one. But he was a practical man and as well for fighting as for holding a bottle of sake one arm was enough. Even though he was left-handed before and now right-handed necessarily.

Only sometimes he cursed the fact he didn't have his second arm anymore. He didn't regret it, no way, but a lot of things were harder and needed more finesse, more effort and as everybody knew, he was too lazy for that.
It was simple things. Open up a lock, for example. There wasn't a second hand which could pull or hold it, when you didn't get to open it up.
Shanks still believed the storage room with the sake in it lacked some locks since then, but instead had some which clamp more. Benn was pretty adamant with that.

But he was also a big help, back then as he was now. His first mate changed a lot of small things to make it easier for him without Shanks needed a word of explanation.
After the incident, he never had to fill up his inkpot for example, to write into the logbook, which was after all a task only the captain did, rarely the vice. The quill was always sharp, the book never knotted.
Tie off knots was another story which was pretty hard with only one hand. In fact Shanks found himself some pretty effective tricks, nonetheless he found the ropes always in perfect order, since they left foosha village. Ben was neat, as always.

While thinking about it, he found more and more things Benn arranged for him, to make it easier. Some of them he was never aware of, some just fall in place by the daily routine. Because not only Benn, but his whole crew, this Shanks was for sure, were proud of his captain and his deed he did back then. Especially when they hold Luffy's wanted posters in their hands, which showed a higher bounty every time. How much was it now? 400.000.000 Berry it must be.

Luffy rumbled through Marineford pretty badly. He earned his bounty even though it was painful that he couldn't save Ace. Even Shanks hit the poor boys fate, had he guessed, no, had he knew that it wouldn't come out good if Ace kept on chasing Blackbeard. He didn't meet Whitebeard for nothing. But it was history, two years passed and it was nothing he could change now. He liked Ace, only for coming to thank him for rescuing his little brother.
Shanks really had granted Luffy the rescue of Ace, but know it was unfixable.
Past was past, because you couldn't change it. The war of Marineford was now a memorial for everybody. Yes, he could have seen Luffy then, but it wasn't the right time for it.

He gave Luffy the straw hat with a promise and he would not break it by seeing him premature.
It still surprised Shanks when he thought about how alike Luffy and Roger were. His former Captain, the Piratking of the old time and his protégé, who in his eyes would be definitely the Piratking of the new time.

"You don't have what it takes to be a pirate."
"Yes, I do! One day I'll find myself a crew that is as strong as yours! And then I'll find the world's biggest treasure! And become the King of pirates!"
"Oh .. so you want to become bigger than us, huh? Well then, this hat is my gift to you! This is my favorite hat, you know. When you become a great pirate in the future you'll return that hat to me."

He already knew at that time that Luffy would become someone big, but today, after all the boy had done, Shanks had no more doubt about it. Alabasta, Enis Lobby, Impel Down, Marineford. Rumor says he was at Dress Rosa at the moment. He would reach his goals there, as well.
Everything Luffy did filled Shanks with pride. What was an arm against all of these achievements? Nothing, absolutely nothing. Pain went by, it was long gone since the scar last hurt and the pride, yes his pride outlasted all of this. Until today, every time he thought about the little rubber boy from the small foosha village, Shanks' chest overflowed with pride.

Actually this was a reason for partying. We would arrange with the locks one day. After all Benn never denied him a party when he wanted one, even though he couldn't open the locks by himself now. A small loss in consideration of the income.