Occasionally I'll encounter a story where the protagonist is pleasantly shocked to find a random girl who has mysteriously became his girlfriend. Explanations for this have included things like mind control over the girl or that the girl is from another dimension where she is the protagonist's girlfriend. So, I propose this variation: The girl (and those who confirm her story, if any) are lying. The reason he she is suddenly his girlfriend that morning is because last night, she snuck into his room and this morning, she claimed she slept there. A reasonably friendly motivation for this is that the girl is sick of the fact that the guy likes her but isn't making any move and this approach lets her get to the juicy stuff (e.g. sex and living together) without all the actionless dating beforehand. I'm not sure which perspective makes for a more entertaining story, but this is not something I'll write.