Ghosts are always situationally dependent. This is the key to understanding why someone believes they saw/heard/felt one. I'll list causes starting from the easiest and obvious first continuing through the more esoteric.
1. Power of Suggestion: A place has a reputation for being haunted and those who experience a ghost are familiar with the stories. Or a building looks "creepy" or seems like it
should be haunted. Then there are cases where someone just follows the crowd and talks themselves into believing they've seen a ghost.
2. Hysteria: This one is simple. You hear something moving in the house at 2 a.m. At 2p.m. it's the cat, or the wind knocked something over on the back patio, or heating duct/ plumbing rattling. At 2 a.m. it's either a burglar or the Clown from
IT. You grab a baseball bat and search the house and find nothing and go back to your room. Ten minutes later you hear more noise. You've already searched the house and checked the doors and windows. If you are prone to believe in ghosts your mind will always go there first.
3.Cultural Indoctrination and or Peer Pressure: Fact is that there are groups of people who take the existence of ghosts without question. Don't even waste your time trying to argue with them. The other condition comes when someone who has prestige or charisma exposes their belief in ghosts, and those who want to curry favor follow suit without asking questions.
4. Twilight Sleep: This stage is between a waking state and sleeping state where REM is possible without the sleeper's knowledge. 50% of all reported ghosts come during Twilight Sleep.
5. BS: This may come as a shock but some people like to lie for a variety of reasons. Pro Ghost Hunter Tip: if a Bed & Breakfast invites TV ghost hunters to investigate their property there's a 99% chance it ain't haunted.
6. Infrasound: This is the 500-pound gorilla of the Paranormal Experience world. Simply put, this low-frequency sound wave WILL make you feel like you're being watched which leads to hysteria.
This comes out of work done in 1998:
http://www.richardwiseman.com/resources/ghost-in-machine.pdf
These links also explain the phenomenon:
https://science.howstuffworks.com/s...erceptions/infrasound-paranormal-activity.htm
https://curiosity.com/topics/got-a-ghost-problem-it-might-just-be-infrasound-curiosity/
https://gizmodo.com/some-ghosts-may-be-sound-waves-just-below-human-heari-1737065693
This one dovetails with my next point:
https://interestingengineering.com/tuning-ghost-frequency
7. Involuntary Neurological Response to Latent Stimulus: Putting this into English; you have 5 senses that are much more sensitive than we expect. Our 5 senses work in concert to give our brain information about our immediate surroundings. Sometimes this information is inaccurate and can put pictures or sounds in our heads that are not there. A faint change in room temperature, humidity, a faint odor, or a familiar sound can light up parts of your brain. Most of the time your brain does a quick systems check and double check and blows it off. But once in a while your brain will run with it.
Let me explain. There are certain scents that evoke strong emotional responses in people. For example the smell of buttered turkey mixed with mashed potatoes invokes powerful memories of Thanksgiving holidays. A song can come on the radio and I can recall specific events which occurred in my past where that song was on in the background. People often respond negatively to the "Hospital Smell" because nobody goes to the hospital when things are great. Each of these responses come with a cascade of emotions and mental images. The difference is that we're not always aware of the input causing the response.
8. Bad Construction Practices and Poor Construction Materials: My first four paranormal investigations taught me a lot about plumbing and heating. Unsecured pipes will rattle and bang inside walls or against floor beams. In the early 2000's reports of brand new houses being haunted went through the roof with tales of Indian burial grounds and slave cemeteries being desecrated. The truth took a few years but turned out to be devastating: Poisonous Chinese Dry Wall.
https://www.npr.org/templates/story/story.php?storyId=114182073
https://www.cpsc.gov/safety-educati...how-can-i-tell-if-my-home-has-problem-drywall
https://www.consumerreports.org/cro...all-causing-home-and-health-hazards/index.htm
The Chinese dry wall signs match up with some big haunted house signs: the smell of sulfur and electric lights turning on and off for no reason. This one came out of the blue and many sleaze-ball ghost hunters took advantage of people.
9. Carbon Monoxide Poisoning: The first thing a good ghost hunter will do is check the CO level in the house. This also falls under poor construction practices but can happen in any home with a gas heater. Over time a person living in a house where CO levels are high will hear voices and sometimes hallucinate. Eventually the CO will kill them but this is dependent on the time of year. Many states now require home owners to by smoke alarms that are also CO detectors.
10. Physiological Changes: Around the time I saw a black hooded figure at the foot of my bed I had become a diabetic and suffered from Sleep Paralysis. Luckily because I was a ghost hunter I knew the image was in my head, but damned if it wasn't impressive anyway. This is when people who didn't believe in ghosts start seeing them. See your doctor and have a comprehensive lab test done.
There are certainly more causes behind why people see ghosts, most of those are related to the effects stress and surprise will have on perception. This is why in crisis situations people tend to focus on random things and relate them to the larger event even though they had nothing to do with the event (we see this with conspiracy theories but this has also put innocent people in prison as well).
What I'm waiting on is research about how smart phones are effecting human perception and reasoning abilities. The rise of reported hauntings seems to merge with the introduction of the smart phone and I'm betting there is a direct link.
I'll end with this. The thing that changed my mind about ghosts is the fact that the most common ghost is that of a still-living person. I've lost count of how many people swear that they heard their wife, husband, child, or sibling somewhere in the house only to discover they were not there. The only solution to this event was that it was all in that person's mind. The fact that it was real to them suggests that sounds and smells can create a false impression because the sounds and smells are familiar coming from an expected place. Your imagination does the rest.
trying to free myself from superstitious/magical thinking. 
