Deception is necessary in these relationships in order for the characters to get along. Titania has to tell Oberon that she is not cheating on him, and Oberon has to tell Titania that he is not cheating on her. If the two are fighting, it is bad for the forest.
Titania explains to Oberon that the two of them fighting is killing the forest. It is better for them to make up and pretend everything is okay.
Therefore the winds, piping to us in vain,
As in revenge, have suck'd up from the sea(90)
Contagious fogs; which, falling in the land,
Hath every pelting river made so proud
That they have overborne their continents. (Act 2, Scene 1)
As in most couples, she blames him and he blames her. However, they are better off to kiss and make up, and they both realize it.
The situation is less dire in Demetrius and Helena’s case. Demetrius is in love with Helena, but Hermia’s father wants him to marry his daughter, and apparently Hermia is a better catch. Lysander uses Demetrius’s fickle behavior to describe him as “spotted and inconstant.”
Demetrius, I'll avouch it to his head,
Made love to Nedar's daughter, Helena,
And won her soul; and she, sweet lady, dotes,(110)
Devoutly dotes, dotes in idolatry,
Upon this spotted and inconstant man. (Act 1, Scene 1)
In the end, Demetrius does end up with Helena. Perhaps he lied when he said he didn’t love her, to spare both of them the grief. After all, Demetrius and Hermia’s wedding would have been a business transaction, and a marriage of convenience, not love.