When you're stranded in Equestria. Stuck in the body of an alicorn, but being uncapable of using your powers. And when even the smallest thing that has significance that you do, isn't remembered by anypony. And when nopony can even see that you're an alicorn.
Then what do you do?
You kidnap the new princess in a desperate struggle for attention.
The fact that the one who gave you the powers, namely Discord, isn't really helping you in this is another factor.
Note: shipping will come, someday, just not today.
Note 2: this story began before i knew that season three would feature a Twilight turned alicorn too, so this idea is somewhat original (i hope).
Note 3: Because even my prereader missed it, the main character is a brony.
I gots a prereader: Tsaukpaetra