The Strings inside people are mentioned several times in the novel Paper Towns written by John Green, the Strings are the metaphysical strings within you, there are more than a few metaphysical strings in your soul and mind. Sometimes, all of of your strings break and that usually leads to Depression, Suicide, Robert Joyner's strings broke and he killed himself with a gun.

See his Omnictionary page here:

Though, Truthfully you Can't live Your Whole Life without at Least One of Your Strings Breaking

Their the Metaphysicals in your Mind and Soul that keep you Together and Well.