Thread breaking can be lots of stuff.
Burrs on the throat plate or hook.
Bad needles - they can be deformed from the factory, be bent, bad points, hitting on something.
Tension too tight - upper or lower
Thread winding around the spool pin.
Thread catching on the spool.
Dried up oil on the tension.
Dried oil or burrs on a thread guide.
Rotten/cheap thread.
Bent or rusty bobbin.
I'm sure there are other things can cause thread to break.