Why did Shanks ask Whitebeard to stop Ace from going after Blackbeard?

[Warmmedown]

In chapter 434.

Was it because he knew Ace was Roger's son? Or that he knew Ace's relationship with Luffy? Or just that he liked Ace? Or because of something else? He mentions Ace's fame and trust complicating the issue. What does that mean?

 

[SSJ2]

Ace and Shanks seemed to be on good terms with each other, and Shanks was aware that he was Luffy’s brother. I think his point was that even though Ace was the famous 2nd division captain of the WB pirates, he was still going to lose to Blackbeard.