The nucleophile, being an electron-rich species, must attack the electrophilic carbon from the back side relative to the location of the leaving group. Approach from the front side simply doesn't work: the leaving group - which is also an electron-rich group - blocks the way.

What Orbital does a nucleophile attack?

The carbonyl carbon is the electrophilic atom. The HOMO in the nucleophile is the orbital that contains the lone-pair electrons. The HOMO of the nucleophile approaches the LUMO of the carbonyl carbon at an angle of about 105° to get maximum overlap. The carbon atom in the adduct ends up with tetrahedral geometry.

Where does an electrophile attack?

An electrophile is something which is attracted to electron-rich regions in other molecules or ions. Because it is attracted to a negative region, an electrophile must be something which carries either a full positive charge, or has a slight positive charge on it somewhere.