Your melody will be catchier if you repeat it. Your melody will be catchier if you repeat it. Your melody will be catchier if you repeat it.
See what I did there?
Repetition is a powerful tool, both in the melody and the rhythm.
This is why the cadence/phrasing of the lyrics needs to be somewhat consistent.
For example, let’s look at The Beatles’ “Here Comes The Sun.”
The phrasing of the melody is consistent from one line to the next (in each respective section). The melody also repeats, but not too much that it gets old.
The bold lyrics below have the same melody and phrasing. The italicized lyrics have slightly different melodies, and the second line has one less syllable:
Little darling, it’s been a long cold lonely winter
Little darling, it feels like years since it’s been here
Similar enough that it’s memorable, but different enough that it’s interesting.
If you have to, I’d suggest counting out the syllables in your lyrics.
You may think I’m being nit-picky, but focusing on repetition — in melody and lyrical phrasing — will go a long way.