In a song with only one melody part I often find it boring to hear that same 8-bar melody repeat, verse after verse. If I don't know the B part of the melody, I will often compose a B part, either for the chorus or to alternate between verses. B parts are easy to compose as they are sort of turn-arounds to the A part.

There's often a structural relation between dance tunes - 8 bars of the A part, often repeated, followed by 8 bars of the B part, again repeated - and song melodies. Not surprising since dance tunes were/are often used for song melodies. That 8-bar phrasing seems most natural. Of course there are also "crooked tunes" with a measure added or subtracted, or changing time signatures.

A little music theory can't hurt - just makes things more interesting.

Cheers, S. in Seattle