Why Pranayama with breath retention should be learnt only under an expert guidance?

Pranayama is known to accentuate what’s in our mind. If we are not disciplined, this practice of breath retention will bring out the negative thoughts to fruition. There are clearly dangers to learning pranayama with breath control without expert coaching. However, Anuloma Viloma pranayama without retaining breath, can be practiced by anyone with positive effect on health & well being.