Recently I have read some non-technical publication and books on strings (ex. The elegant universe by B. Greene). Greene says that the string theory unifies all interactions and explains their nature. In fact I haven’t found any explanation why do particles / strings interact. I mean – why do two electrons repel or why do 2 quarks attract strongly? Or why do all particles attract gravitationally? Does the string theory realy clarify the nature of interactions or is it only the mathematical tool for calculations as the quantum mechanics is?