Titania and Oberons conflict are over the changeling which Oberons wants as a slave. Titania refuses to give him the changeling. The argument caused nature to react violently altering the seasons. Oberon wants to put a love potion on Titania so he can capture the changeling while Titania is distracted.
Oberon sympathises with Helena and orders puck to put love juice in Lysanders eyes, puck puts it in the wrong lovers’ eyes and causes conflict. Oberon commands puck to put the juice on Titanias eyes so she will fall in love thus forgetting the changeling.
Puck, being the trickster (2, 1,144), amuses himself by transforming bottoms head into an asses head. He plans to please Oberon by making Titania fall in love with an ass (3.1.1). The mechanicals see bottoms as an ass and all run in terror away from bottom, thus leaving the play unrehearsed.
Oberon is King of the Fairies, master of Puck, and husband of Titania. He needs an heir to his throne which may be a reason he wants the changeling child that Titania refuses to bestow onto him.
Oberon is jealous of the changeling after he sees the attention that Titania gives the boy. Moreover he sees Helenas persistence, as well as the neglect and rejection, Oberon feels sympathy for her as he is experience the same emotional pain she is. Thus he orders puck to assist Helena.
Lysander and Demetrius both love Hermia even though Lysander is her true beloved, however with the potion put into Lysanders eyes, Lysander loves Helena. Hermias and Helenas friendship becomes destroyed because she is taking her beloved