Why the title?

It's a great title, unforgettable.

I think it is quite an imaginative and apt title. Its the demons(the conflicting elements) in Jessica's head saying "let's mess her up".

It is a gem. I was very young the first time I saw it, but I knew it was a great atmospheric classic and would still be unsettling to me as an adult, which it is. I think the vampire, and perhaps the town vampires, had a great time scaring her to death, like a cat with a mouse, they (or her mental illness) toy with her.
also it is a 70's time capsule of a movie.

The movie is aptly named. It's an excellent title for this movie. Solving the mystery of "who" is scaring Jessica means you understood the movie. She's mentally ill & still unstable. For example, schizophrenics perceive their own thoughts (and often when they are conflicting thoughts) in their heads as "other voices" they are hearing. Normal people can discern their inner thoughts and voices as their own and not other beings telling them stuff. Treatments (that actually helped the real organic illness) for mental illness in 1971 were dismal, if not altogether non-existent. Verbal psychotherapy and sedation was the norm. Doctors didn't even know that some mental illness was actually caused by real, organic imbalances in the brain. It's not like today, and certainly no one, the patient nor the supporting family, was commonly, routinely, nor matter-of-fact told that those voices were really their own disassociated thoughts. Mental illness and talk about it was shunned. The family and friends portrayed in this movie appear to be quite supportive for the most part. This movie is not a horror movie. It's a psychological thriller. And people today need to stop taking everything they see in the arts so literally. The character of Jessica is mentally ill and the story is presented through her point of view. Hence her voice over. She's telling you what she's thinking, seeing, feeling. And she's crazy! Her husband tries to shut down anyone who might upset her. He's also human and worn down. He's given up everything for the health of his wife. Jessica lets us know early on that she's not going to share some of her perceptions because it'll clue him in that she hallucinating again. He calms her when he, too, sees someone upstairs and doesn't want it to set her off when it's real. Jessica is scaring herself into a frenzy, to a possible death all alone on the lake. There's only so much the people around her (the ones who even legitimately care about her) can do to dance around her insanity and keep her comfortable. They thought the country would be quieter and less trouble for her, but she can't run away or hide from her own head. The last shot and voice over serve to explain her desperation, loneliness and isolation from the world and everyone, her broken spirit because of the anguish caused by her mental status.
Very little you see in the movie can be taken literally.