Why is Macbeth addressed as Thane of Cawdor by Ross in Shakespeare's play Macbeth?I'm in grade 11 and we're studying Shakespeare's Macbeth. This...

The simple answer is that the king, Duncan, has decided to
reward Macbeth's loyalty and courage on the battlefield by awarding him the title of one
of the slain traitors--The Thane of Cawdor.  Ross
says:

The
king hath happily received, Macbeth,

The news of thy
success; and when he reads

Thy personal venture in the
rebels' fight,

His wonders and his praises do
contend

Which should be thine or his. .
.

And, for an earnest of a greater
honour,

He bade me, from him, call thee Thane of Cawdor. .
.

This is a great honor, but
it is more than that to Macbeth.  It is this fulfillment of part of the prophecy just
delivered to him (and Banquo) by the Three Witches. And it is this fulfillment of the
first part of the prophecy that makes the idea of killing Duncan in
order to fulfill the second part -- that Macbeth shall be king --
so possible in Macbeth's mind.

It is also ironic that
Duncan bestows the title of Thane of Cawdor on Macbeth, since the slain Thane of Cawdor
that Macbeth replaces was killed in rising up against his king, Duncan, in a failed
attempt to gain Duncan's power for himself.