Thanks for your question. A guitar string (or string from any string instrument) will break if it is under too much tension.

As a violin player I've broken many strings while trying to tune my instrument and winding the pegs a little too far.

Remember that the reason we create tension on the strings is to cause their nice sounds. This sound comes from the 'resonant' frequency of the string which is controlled by a few factors including:

-string length, amount of applied tension to the string, string weight, and the flexibility of the string.

These factors are considered when finding different strings for to use for different ranges of notes. And a string just like most objects will break when you apply too much tension. Just keep in mind what would happen to you if you had too people pulling each of your arms in opposite directions. Yikes!