Traditional Aspects: What's the Explanation?

I had a poster ask about Picatrix, Liber Atratus Greer & Warnock, trans page 40:

"The sextile is an aspect of moderate friendship, because the signs that aspect each other by this aspect agree in their active but not passive nature..." OK - so far, so good....the opposition is an aspect of perfect hatred, because the signs that so aspect each other are contrary in all four natural qualities".

They found this confusing because this isn't true of the opposition!

Ok, one of the things that will really frost your noodle is that there is no ultimate, 100% authoritative traditional source on pretty much anything. Sources will contradict each other and be internally inconsistent. Some times parts of traditional (and modern!) sources are just wrong.

In terms of the reasoning behind the nature of the aspects, everyone agrees that the sextile and trine are benefic, the trine being the best. Square and opposition are malefic, opposition being the worst. Conjunction is not technically an aspect, because aspect means seeing or looking and when planets are in the same degree they don't see each other. Classic traditional info which you learn and then disregard since a conjunction in practice is treated like an aspect and is good with good planets, bad with the bad planets.

Ok, now why? Picatrix is giving a fairly standard traditional explanation, here's one from the 17th century English astrologer William Ramesey,

But you must note that application by sextile or trine is good, for they are Aspects, of love, amity and Friendship; yet the trine is more forcible and better, for that a Planet in trine with another is out of houses of the one and the same nature, as fire with fire, earth with earth, water with water, and ayr with ayr.
A sextile is by a fiery with an aery, or earthy with watry, and therefore is not altogether so good as a trine, for that these agree not in every point; for fire is hot and dry, ayr hot and moist, agreeing partly, viz. in heat: so likewise earth and water; the earth is cold and dry, the water cold and moist, agreeing in coldness; for which reason a sextile is accounted indifferent good,

Astrology Restored/Astrologia Restaurata, page 92

Let's look at some examples, if a planet is in Aries and aspects a planet in Leo, they are trine (we are going to be ignoring out of sign aspects for these examples). Aries is a fire sign, thus hot and dry, Leo is a fire sign, also hot and dry, thus they agree in both elemental qualities.

Ok, Aries to Gemini, a sextile, fire = hot and dry, air = hot and wet, they agree in one of two elemental qualities.

Aries to Cancer, square, Aries = fire = hot and dry, Cancer = water = cold and wet, they agree in no elemental quality.

Ok, so far so good! Aries to Libra, opposition, fire = hot and dry, air = hot and wet, agree in one elemental quality, OPPS! the sequence breaks down!

In fact, the basis of the aspects is not elemental qualities, but rather the traditional planetary rulerships of the signs. This everyone agrees on.

Start with Leo and Cancer each ruled by the Sun & the Moon, one sign away is Gemini & Virgo, ruled by Mercury, not an aspect but Mercury is changeable anyway, good with the good, bad with the bad. Two signs away, a sextile!, are Taurus & Libra, ruled by Venus, the Lesser Benefic, 3 signs away, square, are Aries and Scorpio, ruled by Mars, the Lesser malefic, 4 signs away, a trine, are Sagittarius and Pisces, ruled by Jupiter, the Greater Benefic and 6 signs away, the opposition, are Capricorn and Aquarius, ruled by Saturn, the Greater Malefic. Check out the handy diagram at the top left of this post.

This explanation is internally consistent and accepted by every traditional source I've seen even if they have alternate explanations elsewhere.