There are a variety of reasons why the thread on a sewing machine may break. Some of the most common causes include:

1. Incorrect thread tension: If the tension on the sewing machine is too tight or too loose, it can cause the thread to break. It is important to adjust the tension according to the type of fabric and thread being used.
2. Poor quality thread: Using low quality or damaged thread can increase the likelihood of the thread breaking. It is important to use good quality thread that is appropriate for the sewing machine and the type of fabric being used.
3. Incorrect needle size: Using the wrong size needle can cause the thread to break. It is important to use the correct size needle for the type of fabric and thread being used.
4. Frayed or knotted thread: If the thread is frayed or knotted, it can increase the likelihood of the thread breaking. It is important to cut the thread cleanly and avoid knots in the thread.
5. Old or worn sewing machine parts: If the sewing machine is old or has worn parts, it can cause the thread to break. It is important to properly maintain the sewing machine and replace worn parts as needed.

To prevent thread from breaking on a sewing machine, it is important to use good quality thread and the correct size needle for the fabric and thread being used. It is also important to properly maintain the sewing machine and replace worn parts as needed. Proper thread tension is also important, and it is helpful to refer to the sewing machine manual for guidance on adjusting the tension.