Both Inform 7 and TADS 3 have fairly sophisticated systems for producing
conversation between the player and other characters. (In fact, some of
the better conversation extensions for I7 were written by Eric Eve, who
is primarily a T3 author. He adapted the concepts in the T3 conversation
system to I7.)
These systems will let you do things like menu-based conversation,
suggested topics, and alternate responses based on the state of previous
conversations or other events.
That said, there's a difference between a flexible conversation
framework for a game (which is difficult to write, but very possible)
and artificial intelligence (which is far _more_ difficult).
What I have found is that writing believable, human-seeming characters
in interactive fiction is quite a challenge! The reason a lot of games
end up being dungeon-crawls is because dungeon-crawls are easier to program.
You might want to begin by writing out a dummy transcript. Write a few
scenes as they would appear to the player of your game. And then imagine
what may happen if the player is uncooperative and fails to type the
commands you're hoping for! This might give you some insight into what
sorts of programming tools will be most useful to you.
You might also try playing "Galatea" by Emily Short, which is mainly a
conversation game (though not a romance).
--Jim Aikin