The behaviour of ketones and aldehydes can be easily understood from their structures as shown below.
From these structures two major points come in front of us:
1. The ketones have alkyl groups on both the side of carbonyl group, creating steric hindrance for the nucleophile to attack over there. But in aldehyde , the nucleophile can approach easily.
2. The alkyl groups attached to carbonyl group increase the electron density on it. Due to this the nucleophile feel repulsions while it is approaching near to the carbon atom of carbonyl group.
But in case of aldehydes the carbon atom is having lesser electron density , as it is joined by a hydrogen atom on one side. And also due to the smaller size of hydrogen atom it is easy for the nucleophile to attack on central carbon atom.