In "A Midsummer Night's Dream," who first thinks of using the love potion on Titania?
Oberon, the king of the fairies and Titania's husband, is the character who sends Puck to put the love potion on Titania's eyes. He and Titania have been arguing over a changeling, a boy, which Titania possesses and Oberon desires for his own. He wants Titania to become spellbound, lovesick over some "vile thing," so that he can take the child without her noticing or protesting.
In fact, Titania and her changeling are the main reason why Oberon instructs Puck to fetch the magical flower with the love potion effect. Demetrius is an afterthought of Oberon's after he witnesses Helena's pursuit of Demetrius and his cruel dismissal of her affection. Lysander is a mere innocent bystander.
In Act II Sc.1 Oberon is angry with Titania because she does not give him the "little changeling boy" and he is determined to "torment her for this injury," and it is Oberon who first asks Puck to fetch him the flower,
"the juice of it on sleeping eye-lids laid
Will make or man or woman madly dote
Upon the next live creature that it sees."
In the very next scene Oberon squeezes the juice of the flower into Titania's eyes when she is fast asleep and remarks,
"What thou seest when thou dost wake,
Do it for thy true-love take,
Love and languish for his sake:
Be it ounce, or cat, or bear,
Pard, or boar with bristled hair,
In thy eye that shall appear
When thou wakest, it is thy dear:
Wake when some vile thing is near."