Literary critic Shirley Nelson Garner argues that there are several reasons why Oberon wants to take the Indian boy from Titania, but the biggest reason is that Oberon wants Titania's affection all to himself.
One reason Oberon wants to take the boy from Titania is that, as Garner points out, Titania has an affection for the boy that borders on the erotic. We see this strength of affection when Puck informs us that Titania "crowns [the boy] with flowers, and makes him all her joy," which is the exact same way we see her treat Bottom when she falls in love with him (II.i.27). Hence, if Oberon is recognizing her affection of the boy as being somewhat erotic, then he would certainly feel jealous and want to have her affection all to himself.
Garner explains that another reason why he is jealous is actually because, not only does she feel a somewhat erotic affection for the boy, Titania also had a very close bond with the boy's mother. Titania explains that the boy's "mother was a votaress of [her] order," meaning a devout worshiper of Titania, possibly even a "priestess." But beyond being a devout worshiper, Titania also expresses that she and the boy's mother were very close and very frequently gossiped together and conversed. Thus, when the boy's mother died while in labor, Titania promised to care for the boy. Hence, another reason why Oberon is jealous of the boy is because he was jealous of the closeness Titania shared with the boy's mother.
Therefore, we can see that Oberon is jealous of the boy because he does not want Titania sharing her affection with anyone else and instead wants her affection all to himself.
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 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."