On the battlefield of Kurukshetra, Arjuna dropped his Gandiva and with a bereaved heart, tears-filled eyes and folded hands said – “shall not fight”. Krishna rebuked him – “how has this cowardice entered thee?”
Krishna is not talking about the cowardice of not fighting a war here. Krishna is talking about the cowardice of not following through on his duties which Arjuna may have himself selected. Even if he has not selected the duty and karma that has fallen into his lap, he should have dropped his Gandiva not on the land of Kurukshetra. He could have shunned war before war started but doing it in the battlefield is a sure sign of cowardice and stupidity. He should have known before the war started whether he wants the kingdom of Hastinapur or not. He could have taken a vow of non-violence then before the war started. This would have at least saved him the ignominy of cowardice and the sin of killing his relatives.
Similar is the case with people starting a new job. A person should decide beforehand whether this job is for him or not. Showing cowardice after he joins his job is not a nice thing. He should at least try for the respectful exit after fighting and not the cowardice of not fighting just when the job has started.