why did Shanks cried when luffy told him his dream?

In chapter 1060 when luffy tell his dream to strawhats, Robin ask how Ace,Sabo and Shanks reacted to his dream.

Luffy replied they all laughed but Shanks cried. Even the strawhats were either shocked or laughed on his dream. 

So why did Shanks cried?

I have a simple answer for it that is Luffy and Roger dream was same, when Shanks heard Luffy's dream he remembered his captain who was not able to fulfill his dream and died. This made him cry. I don't there could be any other reason. By the way what do you think about it, comment down.

And here is a link to luffy and Roger's true dream theory. I hope you will like it for sure. Link - Luffy's true dream