There are techniques to use to slow down an accumulation of dust and dirt on your DSLR sensor, but if you take enough photos, eventually you’ll need to clean it.
Each time photographers change lenses on their DSLRs they open up the camera to allow dust to enter. In addition, zoom lenses can bring in dust to the camera. Many zoom lenses literally vacuum in dust to the sensor area of the camera when the photographer zooms their lens in and out. Different zoom lenses have different affinities for pulling dust into DSLRs. Consumer quality zoom lenses which use external lengthening/shortening to change focal length are particularly prone to this, compared to pro level zooms which zoom internally.