Life is so much easier with them. We don't have to be for anything, only against. We don't have to worry about earning trust, we instantly have it by choosing a side. We have instant purpose and meaning: to bring down someone or something.
Enemies are addicting. When we don't have natural enemies we will create unnatural enemies, just to make our lives easier.
Of course easier is rarely better. But, it is easier.
But Jesus was always after the better - not the easier. So, he said don't have enemies. Get rid of them.
Now we get to be for something. Now we get to earn trust. Now we get to find purpose and meaning in building up someone or something.
More difficult, maybe. But, always better.