Most common questions used to investigate
Are your guitar strings old and worn out?
Are your guitar strings dirty?
Do your guitar strings break close to the bridge of your guitar?
Do your guitar strings break along the neck of your guitar?
Do your guitar strings break close to the nut of your guitar?
Do your guitar strings break close to the tuning pots at the head of your guitar?
Are your guitar strings breaking over the pickups or where your strumming hand usually is?
Are you using the correct guitar strings for your instrument and particular tuning style?
Common conclusions
Your guitar strings break because they are worn out and weak. Try using new strings on your guitar.
Your guitar strings break because they are dirty and that causes them to become fragile. Try spending a few moments after you play to clean your strings with a piece of cloth or use products designed for guitar string cleaning.
Your guitar strings break because your bridge is too sharp. Try using a file or a piece of sandpaper to slightly smooth the bridge where the string lie.
Your guitar strings break because the edges of your frets are rough. Try using a file or a piece of sandpaper to slightly smooth the edge of the fret where the strings break.
Your guitar strings break because the nut of your guitar is rough or dirty. Try using a file or a piece of sandpaper to slightly smooth any rough parts of the nut and clean the slots in the nut where the strings lie.
Your guitar strings break because the edges of your tuning pots are rough. Try passing an old, thick guitar string through the string hole and move it in a circular motion along the edges of the hole to smooth the edges of the tuning pot string hole.
Your guitar strings break because you may be using the too much force while playing or the pick your using may not compliment your guitar playing technique. Try playing less aggressively and using a different pick.
Your guitar strings break because you are not using the correct strings for your instrument or for your particular tuning style. Try using custom strings designed for your tuning style and make sure you are using the correct strings for your instrument.
Your guitar may have some other problems. Take it to a professional for a set-up
References
https://www.stringjoy.com/why-guitar-strings-break/
https://www.youtube.com/watch?v=riA3y-qckp0
https://www.fender.com/articles/tech-talk/why-do-guitar-strings-break
http://blog.ernieball.com/interesting/how-to-avoid-breaking-guitar-strings/
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