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.