As shown in the two-part examples in 6b, the progession from V to I creates a strong and natural sense of resolution. Why do we hear this?
There are many reasons, but perhaps most importantly, the voice-leading between these chords naturallys resolves from the V chord to the I chord. As we discussed in Unit 2c, it only takes reducing one of the perfect 5ths in the circle-of-fifths by a half-step to create a diatonic collection out of the chromatic collection. When grouped together, the diatonic scales are a collection of whole steps and half-steps, and when you study the examples on the next page, you will see that the half-step resolutions are also the foundation of voice-leading in diatonic progressions. If you understand the voice-leading principles that pull the V chord into the I chord, you can then extend these rules to create the basic progression from which all diatonic harmony evolves.
As a side note, the significance of cultural conditioning cannot be overlooked. A person raised around any style of music will be conditioned to hear the tendencies used in that music as a natural progression, and this holds true for those raised around music descended from the diatonic tradition. This does not change the importance of voice-leading in forming these progressions, but it is worth remembering the difference between laws, rules, and strategies discussed in the first reading from Unit 6.