why the current flow is always oppositte in direction to the conventional current direction ?..

 In metallic solids, electric charge flows by means of electrons, from lower to higher electrical potential.

In other media, any stream of charged objects (ions, for example) may constitute an electric current. To provide a definition of current that is independent of the type of charge carriers flowing, conventional current  is defined to flow in the same direction as positive charges.

So in metals where the charge carriers (electrons) are negative, conventional current flows in the opposite direction as the electrons. In conductors where the charge carriers are positive, conventional current flows in the same direction as the charge carriers.

Availability of free electrons and application of potential difference are the cause for the current flow is always oppositte in direction to the conventional current direction.