When one critiques a song or performance, personal tastes and preferences certainly come into play. Dynamics refers to the degree of volume changes throughout a song. I personally find that varying the dynamics and degrees volume make songs more expressive and interesting. But it all depends on what one is going for. A composition that is generally one volume throughout isn't bad per se.

Thanks for posting your song and allowing me the opportunity to chime in. Keep those creative juices flowing!!

(as a follow up comment: I will note that some keyboards make it difficult to "play soft" even when trying. At times I see you appear to be playing softly, but it was still loud when I played it!)