How does a dominant seventh chord create strong tension before a cadence?

The dominant seventh chord creates strong tension before a cadence primarily through the inclusion of two specific dissonant elements that demand resolution to the tonic chord.1

• Leading Tones: The chord contains the seventh note of the scale (the leading note), which has a strong melodic pull toward the tonic. Additionally, the seventh of the chord itself (the fourth note of the scale) pulls strongly toward the third of the tonic chord.1
• Tritone Dissonance: The dominant seventh chord incorporates a tritone interval between the third and the seventh of the chord. Historically considered a particularly unstable dissonance, this interval requires resolution to a consonant chord, which is achieved when the dominant seventh moves to the tonic.1

By combining these two leading tendencies and the inherent instability of the tritone, the dominant seventh chord functions as a powerful harmonic engine that pushes the music toward a point of arrival. This integration of dissonance into a system otherwise built on consonant triads became a cornerstone of functional harmony.1