It's the nature of the chords you're able to play - you're given the 1-7 chords of a specific mode (the most common ones are the major and minor modes, the notes of those modes make the major and minor scales of a key - but there are several more in this thing which is pretty cool). So basically the only chords you can combine are ones that are in the same key, and they'll pretty much all sound good, just with a different kind of feeling.