Krishna addresses a question that might arise in any mind: why follow shastra at all? Why not just do what feels right? His answer is blunt. The person who throws aside scriptural guidance and lives purely by personal desire attains nothing — no success, no happiness, no moksha. Three negatives in a row, like three locked doors slamming shut.
Kamakaratah — acting on whim. This is not the same as following one's conscience. Conscience is informed by wisdom and tradition. Whim is the raw pull of desire dressed up as personal choice. A traveler who tears up the map and walks wherever fancy takes them may feel free for a while, but they will not reach any destination.
The triple denial — na siddhi, na sukha, na param gati — covers everything a person might want: worldly accomplishment, ordinary happiness, and spiritual liberation. The life lived without guidance from shastra misses all three. Krishna is not demanding blind obedience; he is pointing out that accumulated wisdom exists for a reason, and ignoring it has real consequences.