In Midori Days, Seiji woke up one morning to find his right hand to be Midori. Why is this so? Is it some kind of a curse? Was it explained in the manga or anime why?
I'm not sure if it's explained in the manga but it's sort of like a Pygmalion type of story. Seiji can't get a girlfriend because he's got a reputation as a tough guy and most girls are afraid of him, eventhough he's sort of got a noble quality about it all. So he wishes that he had a girlfriend and it just so happened that a Midori from another school likes him. So like Aphrodite did in the Pygmalion myth, his wish is granted through some divine/magical/freak-of-nature event where Midori's consciousness and likeness was transfered to Seiji's right hand.
There's extra meaning here because, being someone who (according to himself) will never find a girlfriend, the only "girlfriend" he'll have is "his right hand", a euphemism turned to reality.
“Three years high school, and the rest of my life for that matter are gonna end with no girlfriend but my right hand! I can’t take it anymore!”
Additionally, he has a reputation for having a "Demon right hand" through his fighting prowess, and in probably the greatest possible contrast, his "Demon right hand" is turned into Midori (美鳥 beautiful bird). This essentially takes away his ability to punch with his right hand and forces him to change as a person.