Cant take the credit but this a masterpiece of unintentional satire.
Despite his relationship with Luffy, we almost know nothing about the guy. The most mysterious question is why a Yonko caliber pirate would sacrifice his arm to save some random little kid who resembles Gol d. Rogers.
My theory is that the answer comes from the knowledge Shanks learned from being on Rogers' crew.
Roger's an interesting character. He's been mentioned so many times, yet he has absolutely no role in the plot so far. I'm starting to think that Rogers will end up like a Madara-like character, who's manipulation and influence is much more than we would think.
When Roger's found the One Piece he found the secrets of the World and the Ancient Kingdom. However, being ill and on his way to death, he could not challenge the World Government at that moment. But he found a way. Rogers was able to somehow pass his will on so that he will become reincarnated.. He stored massive power and his legacy into the One Piece and made it so that only his successor would be able unlock it.
This explains why he chose to turn himself into the Marines. He used the opportunity to deliver a monumental message to the world to look for One Piece, when in reality this was his way to get his successor to obtain One Piece. What happened to that successor imbued with Roger's will? During Roger's final days, he met up with an inspiring justice seeker, Dragon. Thus he was able to pass his will to Dragon's son, Luffy, which explains all the similarities between the two (Luffy is kind of the reincarnation of Rogers).
How does shanks fit into the picture?
In his time as a Rogers Pirate, Shanks discovered the secret of the One Piece and Roger's plan. His mouth watered at the sight of One Piece's incredible power. The problem was that it was not meant for him and there was no way he couldn't unlock it by himself.
That brings us to chapter 1. Shanks managed to deduce that Roger's will was passed to Dragon's son and tracked him to Foosha. Think about it, why would such an infamous pirate, close to becoming a Yonko, travel all the way to the East Blue and camp in a peaceful village for a year? It's clear that he intentionally came for Luffy. He would use Luffy, as Roger's successor, to unlock the One Piece for him. He used this time to gain Luffy's trust.
As a final trust gaining method, Shanks secretly hired Higuma and his band of thugs to capture Luffy. Shanks then pretended to play the hero and rescue Luffy. However, Higuma somehow manages to "escape" with Luffy to sea. Isn't it surprising how a Yonko level pirate gets tricked by a lowly mountain bandit? Shanks could have easily saved Luffy with Conquerors Haki or found Higuma with Observation Haki. He chose to do neither, because this was his plot. Then, the Lord of the Sea, who Shanks had tamed, comes out and eats Higuma and then Shanks' arm. But Shanks had really planned this all out. He could have saved Luffy unharmed, but chose to give up an arm to really deceive and manipulate Luffy. The fact that Shanks chose to use King's Haki instead of killing the Sea king is further evidence things aren't what they appear to be.
So, when Shanks told Whitebeard he was making a bet for the future, he was really making a bet for himself. I also think that Shanks has a plan to get it back. He knows that the only way to reach raftel is to have a poneglyph reader. He deliberately led Nico Robin and the Baroque works to Luffy. Eventually, Shanks will kill Robin once her use is over and take her devil fruit to regrow his arm. The marineford war is self-explanatory. Shanks saw Luffy in danger, because he knew Ace was Luffy's brother and Luffy was heading out to Marineford. He came to Marineford really to save Luffy.
At the end of the Yonko Saga, Shanks will aid Luffy against BB or Kaido and fake his death.
When Luffy finally reaches Raftel and unlocks the Poneglyph, it is revealed that Shanks followed Luffy there and he steals the One Piece and escapes. The rest of the series is a huge that concludes the marines and the world government ending with Shanks' defeat.