Classical guitarThe 7th note to a major triad chord is minor, so creates dissonance, tension, anticipation for harmonic resolution. 7’s are my favorite chords, major and minor.
In jazz, most chords have become 7’s as a music style.

The most common chords are tertian/triad, constructed using a sequence of major thirds (spanning 4 semitones) and/or minor thirds (3 semitones). Because there are 3 third intervals in a seventh chord (4 notes) and each can be major or minor, there are 8 possible combinations…The augmented augmented seventh chord, comprised of a root, a major third, an augmented fifth, and an augmented seventh (that is, a sequence of 3 major thirds), is a rarely used seventh chord because the augmented seventh interval is enharmonically equivalent to one entire octave (3 major thirds = 12 semitones = 1 octave), and is hence perfectly consonant with the chord root.*

Get it?

*See Seventh chord.