Puck sets forth a few of the details of Oberon's and Titania's fight to an unnamed fairy in Act II Scene i:
"For Oberon is passing fell and wrath,/ Because that, she as her attendant, hath/ A lovely boy stolen from an Indian king./She never had so sweet a changeling./ And jealous Oberon would have the child/ Knight of his train, to traces the forests wild./ But she perforce withholds the loved boy,/ Crowns him with flowers, and makes him all her joy" (II.i. 20-27).
Basically, Titania has a young boy who is getting all of her attention like a mother would give and Oberon is jealous. In addition, Oberon wants the boy to be his so he can train him like a soldier in the wild. Later, Oberon tells Titania that all he wants is for the boy to be a henchman (servant) for him. Titania explains that she has claim to the boy because his mother worshipped her but died at child birth. From that point on, Titania has protected and raised the boy in his mother's loving memory. Here we see that Titania loves the boy like a mother, has raised him since he was a baby, and Oberon selfishly wants to take the boy away from her. To Oberon, the boy is not his, so it would be understandable that he wouldn't value the boy like a son. Since Oberon is not the boy's father, however, he doesn't have much of a say as to what should be done with him. As the custody battle ensues, Oberon decides to drug Titania and steal the boy that way.