this is surely the most absurd notion in a rather bizarre movie.
i always felt that she was just pretending and infact biding her time for some sort of opportunity to present itself before she took her revenge. however it wasn't the case and she was head-over-heels in love with him not long after he had raped her. just unbelievable
It's called Stockholm Syndrome and it's very common with kidnap victims, it is a survival instinct that kicks in :
https://en.wikipedia.org/wiki/Stockholm_syndrome
There are tons of movies with this type of premise, if I remember correctly Harvey Keitel rapes Holly Hunter in The Piano and she ends up falling in love with him by the end. In fact there are tons of romance novels and soap operas with that exact premise as well.
In this particular movie it makes a bit more sense than some of the others, as from what it seems, Jodie Foster is completely screwed, has absolutely no options, and can NEVER go back to her regular life. Dennis Hopper is in the exact same boat, so they share that in common now, and have a better chance of having a life staying together than separating.