You know, there's no reason that you have to use dominant chords. Typically, though, they want to go from the Dominant (V) to the Tonic (I). This is due to the leading tone (the seventh) wanting to resolve to the tonic (the one).
That is, in G Major, the dominant is D, which is D-F#-A-C. The seven in G Major is the F#, which wants to resolve (almost always by going up in pitch) to G.
There is a certain amount of tension, though, in that chord, due to the F# and the C (C is a flatted fifth away from F#, the most dissonant interval there is).
So, you can just play D Major (D-F#-A) which sounds less dissonant, and have that go to G Major (G-B-D).
Seventh chords