If your song were a person, would you consider it stable?
Or is it unbalanced...a little off-kilter...troubled somehow?
One characteristic of a strong song is its sense of balance and stability. It's self-supporting. For example, the ideas in the verses tend to support the main idea of the chorus...and every verse supports every other one.
The mood and feel of the song will likely be congruent with the song's message. The melody will likely "match" the words. Perhaps most important, the message of the song will be congruent with your own unique personhood: your true values and dreams.
Of course, no song or person is "stable" all the time. We often feel out-of-whack for one reason or another and our songs often reflect that. In fact, a song is a very appropriate place to express feelings of confusion and anxiety.
At the same time, simply in the act of writing a song, we draw ourselves a roadmap for emotional stability.
It takes effort to consciously choose the right word to complete a rhyme (for example), just as it takes effort to make conscious choices that support our physical, emotional and spiritual health.
But it's worth it!
When the process is complete, we have a strong, stable, true song. A mirror of our best selves.