Minor thirds are a "load-bearing interval", they are crucial because they define the sound of minor chords and provide an emotional quality. As you listen, pay attention to the slightly sad, tense, or introspective feel of the interval and how it immediately suggests a minor sound. You’ll hear minor thirds everywhere , in melodies, chord progressions, riffs, and vocal lines   and once your ear locks into this interval, it becomes much easier to recognize minor harmony in real music.