What is the reason why some committed individuals are afflicted with waswas (whispers/obsessive thoughts) while others are not, and does waswas only affect the ignorant?

Ignorance is one of the greatest causes of waswasah (whisperings/obsessive thoughts). If a person learns what helps him worship his Lord with clear insight and understands the ease of the Shari'ah, nothing of the waswasah will afflict him. Waswasah may also be a psychological illness that requires treatment. One who is afflicted with waswasah should strive against it and disregard it.