A detergent residue, fabric softener, dirt, and grime buildup can cause a smelly washing machine. Additionally, mold and mildew may develop in damp, warm conditions, particularly in the door seal, gasket, and drum. Infrequent cleaning, using too much detergent, and not allowing the machine to air dry after use can exacerbate these issues, leading to unpleasant odors in your washing machine.