disclaimer: i don't own the legend of zelda (someday)
This is the real reason midna became nice.
Here they were the cursed wolf and the twilight imp, in the master sword's grove. The winds that whistled through the tree tops and spun ancient bridges had stopped without reason as the pair entered the grove, now the only sounds that could be heard were the wolf's footsteps thudding against the moss and leaf covered stone and the quiet breaths of an imp deep in thought.
The master sword's magic began cleaving away at the shadow cloak that covered the wolf, sending the imp that was ridding upon his back flying. Returned to his hylian form Link began to draw the sleeping sword out of the pedestal. It was free; the master sword had chosen Link, of all people, as its master. "Pretty cool? Huh Midna" he began frantically scanning the grove in search of his friend. "Midna? Midna this isn't funny. Come out here now! Please?" it was then he heard leaves rustling, gripping his new sword tighter in his grip he edged forward towards the sound. It was then she popped up. "Link? Link!" the imp hollered out only to realise that he was towering over her his sword aimed at her new play thing. "look what I found" Link just glared down at her, his face plastered with a mix of anger, worry and confusion. Midna decided that her companion was too thick to realise what she was talking about "leaves, I found leaves" she then collapsed into the pile giggling. Link just stood over the pile of leaves, the giggles and the imp all he could do now was watch as his friend played like a child in autumn. Leaves flying everywhere he took a moment to realise she shouldn't get all the fun to herself and dived in next to her. The hero of both hyrule and the twilight realm was playing in a pile of leaves next to the princess of twilight who was currently an imp. This was a sight anyone who saw would never forget.
so in conclusion midna became nice because of the brain damage she sustained after being forced away from the master sword