The answer to 'what causes spider' is the presence of insects, their food source, which attracts spiders to your home. Spiders are predatory arachnids that feed on a variety of insects, such as flies, mosquitoes, and other small bugs. They are drawn to areas where these prey insects are abundant, like around windows, doors, and in dark, undisturbed corners of your home.
Spiders also seek out sheltered, dry areas to build their webs and lay their eggs. Cracks, crevices, and cluttered spaces provide the ideal habitat for many common household spider species. Maintaining a clean, well-lit home and addressing any insect infestations can help discourage spiders from taking up residence.