or why does Duke Frederick want the kingdom of Duke Senior?
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The only thing I recall by memory is when Jaques talks about Duke Frederick's conversion in Act 5 of the play.
"After some question with him, was converted
Both from his enterprise and from the world,
His crown bequeathing to his banish'd brother,
And all their lands restored to them again..."
Duke Frederick just wanted all the power for himself which is why he banished his brother Duke Senior, the rightful leader.