Why does Oberon want to take the Changeling boy away from Titania in Shakespeare's A Midsummer Night's Dream?

Oberon and Titania are estranged because Titania refuses
to give up her page boy to
him.

Puck "For
Oberon is passing fell and
wroth

Because that she, as her
attendant, hath

A lovely changeling
boy stol'n from an Indian king.

She
never had so sweet a changeling:

And
jealous Oberon would have the
child

Knight
of his train to trace the forests
wild."

Act II Scene
I

This boy (Indian  href="http://en.wikipedia.org/wiki/Changeling">changeling) and Oberon wants
him for his "knight" and henchman.

Oberon
"I do but beg a little changling boy to be my
henchman" Act II Scene I

Oberon is also jealous
of Titania's attention to the boy and her feelings for the
boy.

Oberon devises a plan to humilate Titania and take her
attention from the boy. (Eye drops form the magic flower)  He says, "And ere I take this
charm from off her sight,

"As I can take it with
another herb,

I'll make her render up
her page to me."