with the context we now have about shanks’ backstory, the line “he acts just like i did when i was a kid” hits kind of different.
luffy declares that he’s going to be pirate king. but that isn’t shanks, at this moment. shanks gave up on finding the one piece in the wake of roger’s death. somewhere along the line, he lost interest in being at the top of the pirate world. it’s only recently that he’s picked up the conviction to seek out laugh tale again.
at this point in time it’s not revealed yet that shanks was literally adopted by roger, so shanks doesn’t say that luffy reminds him of roger, but he tells rayleigh that later on. luffy is in a sense inheriting shanks’ will as well as roger’s — or more accurately, a dream which shanks has, at this point, given up on. there’s almost the sense that shanks was holding on to roger’s will but not inheriting it — stewarding it, so to speak, so he could pass it on to the next generation.
it’s interesting to think of the 20-ish years between roger’s death and luffy’s arrival as a sort of interim generation. the whitebeard generation, i guess? a generation of stewardship, of guarding roger’s will rather than claiming it. it’s not time yet. the central players have yet to take the stage. but take the stage they will — and shanks is setting the scene for luffy.
as for why he decided not to take center stage himself even when he very well could have? idk your guess is as good as mine.