As a paranormal investigator, one of the first questions I'm asked is, "How did the ghost get here?" The answer isn't as simple as it sounds—it requires asking a lot of questions of the homeowner and conducting hours of research. However, it's been my experience that there are generally five reasons why a ghost takes up residence in a home.

1. The Ghost Is Attached to the House.
   Many times previous owners of a home will come back to the home after death. Normally when this happens a lot of paranormal activity will occur in the house. The ghost is could be confused and, in some cases, frustrated because strangers are living in "their" home.
   
   This type of ghost may have come back to the house because that's where they were the happiest. They can cause footsteps and the opening and closing of doors, and will generally seem to be going through the same routine they used when alive. The ghost of a former owner of your home may give you a real sense of a distinct personality being present, and in many cases, may attempt to interact with you in some way. 
   
   Normally, if you just sit down and calmly talk to the ghost and set down some ground rules you and the ghost can cohabit the house quite nicely. For example, you could ask them to stay out of your childrens' bedrooms at night, not to leave the water running, to stop changing the channel on the television, etc. You can also use this technique to nicely ask your ghost to leave your home and cross over to the other side.
   
   
2. The Ghost Is Attached to the Land.
   Much in the same way a ghost can be attached to their home, the same can be said for the land. This can happen when the house the ghost lived in was destroyed or torn down. The ghost does not acknowledge the new house that may be on the property; rather it's remembering their house the way it used to be. This can account for ghosts appearing to walk through walls or doors that aren't in your house, but perhaps were there in the old house.
   
   A ghost attached to the land can be a little more difficult to get rid of, because in its world, you nor your home exist, and if they are aware of the existence of you and your house, it s very rare for them to try to interact with you in some fashion. 
   
   You can try talking to it like you would if they were attached to your house, but you may not have much luck. If you're a religious person, you can ask your clergyman to come out and bless the property and your home; this could put the ghost to rest. However, in many cases, it may be necessary to learn how to co-exist with this type of harmless spirit.
   
   
3. Home Renovations
   I have seen many cases where paranormal activity will suddenly appear when the homeowner starts renovations on their house. Home renovations could awaken a ghost that has been dormant for years; in many cases the ghost will be that of a former owner who doesn't want changes made to what they perceive to be their home. In other cases, the ghost will become very excited because the home is being taken care of. 
   
   A renovation ghost that is unhappy with the work being done may hide and/or move tools and building supplies, knock over ladders, try to scare the workmen to prevent them from doing the work, etc. A ghost that is happy the work is being done may also move tools and supplies, and mistakenly scare the workers. Or, alternatively, they may try to help in some way with the renovations. 
   
   In some cases the renovation ghost may not bother the work site at all, but show its pleasure or displeasure in other ways by causing paranormal activity in another part of the house. They could be doing this to try to get your attention in order to express their feelings about the renovations. 
   
   Generally, once the renovations are complete this type of ghost will simply disappear over time. If a renovation ghost shows up, simply try talking to it and explain why the renovations are being made and that you understand it may be upset by them. Empathizing with the ghost's position could make the paranormal activity slow down or stop completely. You can also talk to the ghost calmly if it doesn't disappear within a reasonable time after the renovations are complete, and ask it to leave.
 
4. Haunted Items
   If the paranormal activity in your home started very suddenly, think about what you may have bought at a flea market, garage sale, or antique store recently. There are cases where a ghost will haunt an item that they loved or cherished when they were alive. A ghost can attach themselves to just about anything, such as jewelry, a doll, chair, sofa, mirror, a piece of clothing, etc.
   
   The first step in determining if you have a haunted object in your home is to keep track of what room the paranormal activity occurs in most. If it's the room that contains the object you recently bought, chances are there's a ghost attached to that object. 
   
   If you've determined you have a haunted object, the only way to make the ghost go away is to get rid of that object. Keep in mind that if you've inherited some items from a loved one who's passed away, it could be that their spirit is attached to one or more of those items (such as a piece of jewelry). In that case, you would know who the spirit is, and may choose to keep the item and live with the spirit of your loved one.
 
5. Haunted People
   Although rare, it is possible for a person to be haunted. By this, I mean that ghosts tend to gravitate to these types of people. If you've moved several times because you believed your house was haunted only to find out that your new home is haunted, it could be that rather than the houses being haunted, you are. There is a theory among many paranormal researchers that certain people project a particular type of energy that attracts ghosts. Unfortunately, there is no way to really change your energy. People who have this type of energy must learn how to deal with the ghosts that come to visit, and firmly but calmly ask the ghosts to leave them alone.

You can learn more about how a house can become haunted, types of hauntings, and ghosts in my book Is Your House Haunted?.