These words are spoken by the jealous Oberon, king of the fairies, who engages in an argument with his estranged fairy queen, Titania. In Act II, Scene 1, Oberon is engaged in a custody dispute with his queen. a dispute that has little to do with passion as the king and queen of the fairies are accustomed to sexual betrayal--Oberon has given his attentions to "amourous Phillida" and "the bouncing Amazon," Hippolyta and Titania her love to Theseus; Instead, Titania's possession of the changeling boy, the son of an Indian king, undermines Oberon's position.  

Renowned Shakespearean critic Harold Bloom observes that the argument concerns "the protocol of just who has charge of the changeling human child."  This question, Bloom contends, 

...turns upon the question of the links between mortals and immortals in the play....To exclude Oberon from the child's company is...not just a challenge to male authority, it is a wrong done to Oberon, and one that he must reverse and subsume in the name of the legitimacy in leadership that he shares with Titania.

As Oberon says, Titania's hold of the changeling is an "injury," for he asks her pointedly, "Tarry, rash wanton: am I not thy lord?" (2.1.64). Further, as Titania observes that Oberon's and her  quarrels have caused disasters in both the natural and the human realm, he asks her,

Do you amend it, then; it lies in you. 
Why should Titania cross her Oberon? 
I do but beg a little changeling boy, 
To be my henchman. (2.1.119-122).

Oberon wants the boy to be his personal attendant, a page of honor," but when she refuses, he promises her torment for denying him this assertion of sovereignty. That is, Oberon feels he must punish Titania for challenging his authority and leadership legitimacy.