Why does she live the same day over and over again?

its never explained in groundhog day either

Such things are never explained in these types of movies.

In a scene cut from Groundhog Day a jilted ex-girlfriend is shown placing a curse on Phil.

Well, it's a purgatorial experience (of sorts): she has to repeat her final hours over and over again until she finally makes the 'right decisions' and rests in peace (or goes to heaven, or whatever).

Movie Magic!

There's this really cool story I found thru Reddit by a poster who claimed to live an entirely different life with a wife and a young boy. This all happened, unbeknownst to himself right after he was involved in a small bumper collision with another driver who sucker punched him and knocked him out cold. In his other life, he got married and had a young son who grew up to about 8 or 10 yrs of age, but towards the end he started to notice that a lamp in his living room was shaped very odd and peculiar and it made him sit on the couch and stare it for hours on end. His alt-wife got concerned as his obsession with the lamp had him sitting on the couch and staring at it for days until suddenly the lamp started shining brighter and he came to from being knocked out and losing some teeth. To this day, the poster says that he recalls the years he had with his wife and the years watching his son being born and becoming a child but this would all happened in a matter of 5 to 10 minutes but in his unconscious state it was at least a dozen years.