I think it is considered perfectly coalescent and agreeable to the ear. Some intervals are dissonant (sounding out of tune or nasty!)but the 4th and 5th are considered to be the most "perfect" and consonant sounding, after the octave. Also, 4ths and 5ths cannot be major or minor. All the others can. Since they cannot be tampered with in this way, they are just "perfect"!