There is something beautiful about simplicity in music. While most everything in life thrives on complexity, some of the most beautiful music is that which is the most simple.
And you've done well capitalizing on that here. You've got a pleasant chord progression running throughout to create a mellow background. On the foreground, you have a simple melody in the piano's upper register complimented by some chordal block accompaniment in the strings. It all fits together to form a sweet, musical picture.
That being said, though, music also requires variety. You've got a nice start, but the song is pretty static. The chords remain constant, and there are really only two melodic phrases. The only aspect adding a bit of musical variety comes with the introduction of the strings, but I do not believe that is enough. I suggest that you take what you have here, and transition into a new melody with some new chords for a few bars. Maybe you could take what you've got here, and simply change it to a major key. (I don't know how good your music theory is, though, which is important if you do this) You could modulate up a half-step, change the time signature, or just play a few measures with a different accompaniment pattern. (Example: The same chords, but make them block chords and take them and the melody up the octave and slow the tempo)
There are thousands of ways you could add variety, but these are just a few suggestions. You don't have to do anything over the top, but a little change up would be nice. I really like what you've got going here, though! Keep up the hard work. :D