Oberon gives Titania the antidote to the original "love drug" that Puck dripped over her eyes.
While she was artificially infatuated with Bottom, she gave away the child that she had been fighting with Oberon over. He had sufficiently humiliated her (although she didn't know it) through Puck's actions, so he decided she was ready for the cure. The antidote itself is interesting--he uses "Dian's bud" which he says is powerful enough to stay the effects of "Cupid's flower":
But first I will release the fairy queen.
Be as thou wast wont to be;
[Touching her eyes with an herb.]
See as thou was wont to see.
Dian's bud o'er Cupid's flower
Hath such force and blessed power
Dian is, of course, an allusion to Diana the virgin huntress who protects the chastity of young virgins.As for whether or not she is back in love with Oberon...I think what Jamie says is right. She's back for the moment because the reason for fighting is gone, but will they never have another power struggle? There's nothing in the text to suggest that that will be the case.
I do not believe there is ever a clear, original reason given, but Titania and Oberon have quite a history and a stormy relationship. In 1.2.60-64, the pair bicker over their long history of mutual infidelity:
Ill met by moonlight, proud Titania.
What, jealous Oberon! Fairies, skip hence:
I have forsworn his bed and company.
Tarry, rash wanton: am not I thy lord?
Then I must be thy lady: but I know
When thou hast stolen away from fairy land,
And in the shape of Corin sat all day,
Playing on pipes of corn and versing love
To amorous Phillida.
The references to Corin and Phillida, according to critic Gail Kern Paster, are blanket names referring to generic lovers. Therefore, we can assume that Oberon has had many such discretions.
For her part, Titania is no angel. In 2.1.74-7580, Oberon accuses:
How canst thou thus for shame, Titania,
Glance at my credit with Hippolyta,
Knowing I know thy love to Theseus?
Didst thou not lead him through the glimmering night
From Perigenia, whom he ravished?
And make him with fair AEgle break his faith,
With Ariadne and Antiopa?
Still, the two are undeniably attracted to one another, going to great lengths to show it, though it is often mean-spirited and the chain of events they set off full of mishaps. The pair seem to have a chemistry that is at once attractive and repellent to them both.
Titania is in love with Oberon again because Puck takes the spell away from her. The Spell that was making Titania in love with Bottom is no longer upon her and she loves Oberon.