Hello. The most common reason for bad breath is poor oral hygiene. There is constant accumulation of plaque and tartar which cannot be always fully removed by your routine brushing and flossing.
You have to visit your dentist once in 6-8 months for a routine scaling.
This apart sometimes bad breath can be because of various general health issues. kidney, liver, Gastrointestinal etc.
First line of treatment should be a clearance from the dentist followed by a Gen Physician.