For directed negative edges: Since a negative edge makes that distance appear shorter, the true shortest path could end up not being given as the answer. Hence the problem.

For non-directed negative edges: You could enter an infinite loop trying find the "shortest path" as the negative edge creates a distance that goes to negative infinity.

This occurs because Djikstra's algorithm is a greedy algorithm, as Gustav mentions in another answer here.

For directed negative edges: Since a negative edge makes that distance appear shorter, the true shortest path could end up not being given as the answer. Hence the problem.

For non-directed negative edges: You could enter an infinite loop trying find the "shortest path" as the negative edge creates a distance that goes to negative infinity.

This occurs because Djikstra's algorithm is a greedy algorithm, as Gustav mentions in another answer here.