Major and minor 7th Intervals are Dissonant, as are Maj & min 2nds.
So is the Tritone Interval. Their dissonance seeks to be resolved.
That's why a G7 chord wants to resolve to the C Major chord.
The Intervallic qualities are what dictate the level of Consonance or Dissonance within the structure. Example, a Dsus4 chord; = D-G-A.... will create dissonance and usually want to resolve to Dmaj.