"EVERY art and every inquiry, and similarly every action and pursuit, is
thought to aim at some good; and for this reason (dio) the good has rightly
been declared to be that at which all things aim."

*dio*: hence (Rackham) this is why (Irwin), because of that (Rowe).

How could it be that aiming at *some* good implies aiming at *the* good?
This is not coherent. Irwin has an alternative explanation:

"'That is why' (dio) normally marks a transition from the exposition of
Aristotle's own view to the explanation of familiar facts or commonly shared
views in the light of his own view. Here he refers to the view of EUDOXUS;
see 1172b9."

So I could understand Aristotle as saying:
"Every art and every investigation, and likewise every practical pursuit or
undertaking, seems ACCORDING TO MAY to aim at some good: hence IN THAT
CONTEXT it has ALSO been well said that the Good is That at which all things
aim."

It's not easy to make clear sense of this, and Sarah Broadie's statement "it
is not obvious what the reasoning is in these few lines" is
understandable... The NE starts in an obscure way!?

Thomas