Question

why the electic field lines always start from positive charge to negative charge?

Answer 1

It is a convention that has been adopted.

The direction of the field is taken as the direction of a test charge kept at the field. Now if in an electric field generated by a positive charge we keep a test charge (test charge is always positive), the charge will fly away from the source charge. This direction of the test charge the is obviously away from the positive charge and towards the negative charge. This why electric fields starts from positive charge.