It helps me visually when looking at a circuit diagram if I can think about the current flowing "down", like most rivers do in the Northern Hemisphere. It's just a vague association in my mind, but it is based in physical reality.
That convention is the result of identifying current flow as being the movement of that which we identified as positive charge.
It's not a convention that like charges repel.
If we agree that current is the flow of positive charge, then it has to go away from positive, toward negative.
Using positive quantities for current and charge is a good thing; we aren't constantly dealing with negative numbers to measure common situations like how much current is flowing through a wire.
