The textbook explains that aldehydes are more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, ketones have two alkyl groups hindering nucleophile approach, while aldehydes have only one. Electronically, two alkyl groups reduce carbonyl carbon electrophilicity more effectively. For aromatic compounds, benzaldehyde is less reactive than propanal due to resonance reducing polarity of the carbonyl group.