why do electric field lines of force always stay perpendicular to their surface , i mean why do they don't make any other angle with it ?

It's really the other way round.

 

You have a line of force and define a surface to which that line is normal, and passes through. If you defined any slant surface then the effect (integral) over the that surface would be the same. Gauss theorem relates to the projection of any slant surface onto the normal one as equivalent.

Edited September 13, 201312 yr by studiot

why do electric field lines of force always stay perpendicular to their surface , i mean why do they don't make any other angle with it ?

 

I assume you are referring to a conductor. Charges on a conductor reside on the surface. If the field line is not perpendicular, there will be a component parallel to the surface, which pushes the charges around (it's a conductor so they are free to move), and will do so until there is no more parallel force. So in steady-state, the lines must not have a parallel component.