What motivates Oberon to use the potion on the mortals?

Because oberon wants to get the changeling boy.

Explanation:

Oberon instructs puck to apply a charm that will make Titania and Demetrius each to fall in love with the first person to see, this made Titania to fall in love with bottom. who now has the head of a donkey. Oberon is obsessed with changeling the foster child of Titania. Oberon demands she hand over the boy to him but Titania refused. this made Oberon angry, according to puck, Oberon is jealous because Titania give the boy all her attention and ignores Oberon. Puck also suggested that Oberon wants to show Titania that he has power over her. This refusal of Titania to hand over the boy to be his servant which may be a way to show Titania changeling boy motivated Oberon to use the portion.

The little changeling boy Titania got from India, fueled more problems between Oberon and Titania. although their had been problems in their relationship. Oberon wants the boy to be an attendant in his court but Titania refused because she had been taking care of the boy in memory of her friend who died giving birth to the boy. this made Titania spend all her time with the boy and Oberon had to plan a revenge on her to her his way.