Two parallel current-carrying wires - What is the forces acting on each wire?

A short-cut is to think about a couple who are in love or quarrel.

When in love, both are walking together in one direction holding hand (i.e. current in both wires are in same direction). Hence there is ATTRACTION between them.

When the couple quarrel, both are walking in opposite direction (i.e. current in opposite direction). Hence there is REPULSION between them.