An angel's duty is to guard and watch over it's charges everyday. Naturally, every once in a while there is going to be that one human who's life is so enchanting that even an angel wants to be a part of it. So the angel wants so desperately to be in that person's life that they gives up their immortality to live the life of a human, hoping they can find a way to fit in. There are several scenarios for this:
* Female angel sees how much love and friendship goes on in the human world and wants to be a part of that somehow. Either just to be friends with the human girl that she's in charge of or maybe be included in the family somehow. Or befriends human charge and falls in love with the brother/friend of girl, etc...
* Female angel falls in love with human male that she's in charge of and gives up her immortality to join him on earth. Or vice versa: Male angel falls in love with female charge.
* Evil Angel is so envious of their human charge that they actually devise a plan to try to take over their life. They get kicked out of Heaven or go AWOL. Maybe God sends down a few other immortal characters to intervene and end up saving mortals life.
* Devious angel who gets "grounded" from heaven for being mischievous while upstairs. Has to spend so much time as a mortal as punishment, but ends up liking their mortal life better.