I do not believe in ghosts because I have no reason to. The vast majority of ghost "sightings" can be put down to simple humanity. In other words, we look for things we want to see, we are naturally fearful creatures, and we like to jump to simple, convenient explanations even if they may not be true.
Someone in grief may believe they see or hear a dead loved one, someone afraid is constantly looking out for what they fear (and they believe they see it often enough), and generally in these situations these people are unable to think critically and rationally. More rational conclusions for ghost sightings include human psychology, sleep paralysis, or fear of what one doesn't understand.
The apparent ghosts leave absolutely no evidence of their existence, and they are completely immeasurable entities. Thus, for all intents and purposes it's pretty fine to assume they don't exist. I'd certainly be willing to change my stance if sufficient evidence was provided, but I think it's unlikely given there has been, historically, none.
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