On the surface, it seems that Oberon is angry with Titania because she has taken into her care a particularly beautiful Indian boy changeling that Oberon is jealous of because he had his own intentions for the boy. However, literary critic Shirley Nelson Garner points out that there are actually deeper reasons.
We learn of Oberon's jealousy in Act 2 when Puck tell his fellow fairy that "Oberon is passing fell and wrath" because Titania has the lovely Indian boy (II.i.20). Puck further explains:
[Titania] never had so sweet a changeling
And jealous Oberon would have the child
Knight of his train, to trace the forests wild. (23-25)
Garner argues that both Titania and Oberon must feel a physical attraction for the boy, which partially explains Oberon's jealousy. Hence, we can say that one reason why Oberon is angry is that he is jealous of Titania for having the boy and wants the boy for his own care. However, the deeper reason for his anger relates to Titania's own sentiments towards the boy.
We learn that, not only does Titania feel a strong physical attraction for the boy, she also shared a very strong emotional bond with the boy's mother as well. We learn of her physical attraction for the boy from Puck's description that she "[c]rowns him with flowers, and makes him all her joy," which happens to be the exact same way she treats Bottom later on (27). We also learn of her bond with the boy's mother through Titania herself. Titania tells Oberon that the boy was not stolen. Instead, the boy's mother was a devout worshiper of Titania, plus they shared a deep friendship, as we see in Titania's lines, "[I]n the spiced Indian air, by night, / Full often hath she gossip'd by my side" (127). Hence, the reason why she has the boy is that when his mother died in child labor, Titania promised to care for the boy. Therefore, Oberon has several reasons to be jealous of Titania having the boy beyond wanting the boy for himself. Oberon's jealousy can most be attributed to his being jealous of Titania's affection. The first reason why he is jealous of Titania's affection is that the bond she shared with the boy's mother distracted her from bestowing her full affection on Oberon. The second reason why Oberon is jealous of Titania's affection is that her strong affection for the boy is also preventing her from bestowing full affection on Oberon. Hence, ultimately, Oberon is angry with his queen because he wants her all to himself.
Thus, we see that Oberon is angry with his queen because he feels his own physical attraction for the boy and wants the boy for himself, but also because he wants Titania's affection all to himself.
In Scene 1 of Act 2, Robin Goodfellow (Puck), a mishcevous spirit who serves Oberon explains the cause for the fairy king's anger towards his queen. Puck explains that Titania has stolen a "lovely boy" from an "Indian King". This makes Oberon jealous and he tells her that he wants the boy for himself, to accompnay him on his wanderings in the woods. Titania refuses to give up the boy and instead continues to pamper the boy by weaving flowers in his hair. This refusal angers Oberon, and starts the quarell between the them that affects the rest of their fairy subjects.