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. 

The first mention of the changeling boy is Puck who says,

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.

Titania is the next to talk about him when she refuses to give him to Oberon.   Speaking about the boy's mother, she say, "...her womb then rich with my young squire,..."

Oberon is the last to make a heraldic reference when he say, "I'll make her render up her page to me."

It would appear from these references that Puck is referring to the future of the changeling boy, to be knight.

Titania it would seem is referring to the next step for the young boy's progression toward knighthood, that of being a squire where the young boy leaves the company of women to learn the skills necessary to become a knight.

Oberon is referring to his current position as a page since pages were the first step in becoming a knight and they would attend the women and learn the gentle arts from them.

It would appear that Oberon wants the changeling boy because it is time for him to leave the world of women and join the world of men.

The King and Queen of the Fairies, Oberon and Titania, get into an almighty quarrel over a changeling boy from India that Titania has in her possession. Oberon wants the boy for himself, but Titania won't play ball and refuses point blank to hand him over. Oberon claims to want the boy to be his attendant. But in reality, he's just jealous of the lad for spending so much time with Titania. He finds it particularly irksome that Titania crowns him with flowers and "makes him all her joy".

As both Titania's husband and King of the Fairies, Oberon thinks he's entitled to a little more attention from his wife. Hence his cunning plan to get the mischievous Puck to put the juice of a flower on Titania's eyes so that she'll fall in love with the first person she sees on waking up.

Titania's reluctant to let go of the changeling boy for personal reasons. According to her, she was a very good friend of the boy's mother, who was one of her most loyal servants. Sadly, the woman died giving birth to the changeling boy, and Titania, in honor of the woman's memory, agreed to raise the boy on behalf of her late friend.

The new conflict that is introduced into this play by Oberon and Titania in Act II scene 1 is the conflict that they are experiencing. It is interesting that in Act I scene 1, some plays present Thesus and Hippolyta as experiencing conflict, and this is mirrored by the conflict shown in the relationship between the fairy king and queen. The conflict in the case of Oberon and Titania, however, is due to their argument over an Indian boy, whom Titania possesses at the moment and refuses to yield to Oberon, as this quote explains:

The fairyland buys not the child of me.
His mother was a vot'ress of my order...
And for her sake do I rear up her boy;
And for her sake I will not part with him.

This is the conflict that rages behind the conflicts that occur in Athens, and the play of course develops through the intersection of these different conflicts, as Oberon's methods to solve his own marital dispute with Titania end up impacting the Athenian lovers as well. Oberon appears to be quite selfish and petty in his insistent and stubborn demands for Titania to yield up the boy, whereas Titania appears slightly more mature, stating her reasons for keeping him because of the love she had for his mother as justification of her behaviour. It is interesting to reflect however that the fairy king seems to be a character who is driven by human emotions, as what motivates Oberon to punish Titania and gain the boy is greed and jealousy. There is no higher, more moral way of life exhibited by him.