Question

why potential decreases in the direction of the electric field?

Answer 1

Electric potential is work done against electric field to bring a charge from infinity to a point in the field. Now, field lines all go towards infinity. So, obviously, as we bring charge more 'in field', we do more work. In other words, potential decreases along direction of electric field, just because work done to bring a charge from infinity to said point is greater

Answer 2

I hope right!
Such a movement of a positive test charge would be in the direction of the electric field, and so would not need any work. The charge would naturally (intuitively) lose potential energy as it moves through the circuit from the positive terminal to the negative terminal.