In the Hellboy film series (1& 2), Liz has a romantic relationship with Hellboy, becoming pregnant with his twin children. However, in the Hellboy comic books Liz and Hellboy are just friends; there is no hint of a romantic relationship between the two of them and she is more like a little sister than a girlfriend. (Source:Wikipedia)
Did they change this in the movie just for the sake of a love story or any other reason?
With the Hellboy comics, the writers have the advantage of having (as of now) 10 years to be able to write a compelling story with believable characters; with the Movie, the writers are limited to two hours, they have to make you believe that these people are real people with emotions and feelings similar to ours. And it is easier for most people to understand a romantic relationship existing between male and female characters.
Which is another advantage with comic book writers, they know exactly what there audience is, and are able to tailor their story to their audience; whereas movie writers (generally) try to make their movies to be accessible to all people.
According to the DVD commentary for the first Hellboy movie, Mignola also encouraged Guillermo del Toro to do an original story for the film and to do what he liked with the characters including changing relationships.
