She has a Scorpio Sun (obviously . . .), Moon in Aquarius and her S/O is a 29 degree Leo Sun with Moon in Virgo. Her Sun is sextile his Moon and his Sun is opposite her Moon . . . all good, solid indicators for a good energy exchange between the luminaries. However, their Moons are inconjunct so this may be why living together has its glitchy moments. Since his Moon is 1 Virgo, Saturn hit it by transit during the early part of September bringing the differences to the forefront.
Her Mercury in Scorpio is square his Mercury in Leo which leads me to suspect there is a certain amount of drama in their verbal exchanges – and some stalemates as Fixed Sign Mercury’s can be quite stubborn in their opinions. I know, I have one . . . =grin=
There is good sexual chemistry with trines between their respective Venus to Mars exchanges – one in Air and the other in Earth. As Pat Paquette pointed out, good sexual chemistry is very beneficial to a lasting relationship.
Now to some other particulars that illustrate why a deeper analysis can enrich the understanding of the whys. They have an intra-aspect (same planets in aspect between the charts) with Moon to Jupiter. Her Jupiter is conjunct his Moon in Virgo and his Jupiter is sextile her Moon in Aquarius. To me this says they simply like being with the other person. There is an inherent generosity and willingness to give the other the benefit of the doubt with Jupiter/Moon aspects. It is possible they can end up laughing over some point of contention and clearing the air.
For staying power, we look to Saturn aspects and his Saturn is sextile her Sun and conjunct her Mars. Now the Sun/Saturn sextile usually reads as feeling supportive in a friendly manner but the Saturn/Mars conjunction can feel like the Saturn person slams the brakes on the Mars person . . . but it’s also a strong binding aspect. Perhaps that is a factor for Scorp needing some time alone to get a breathing space.
They also have a Uranus/Sun intra-aspect and that REALLY suggests on again, off again. His Uranus is square her Sun and her Uranus is semi-sextile his Sun. This is a setup that is about sudden changes of direction and both of them can pull a classic Fixed Sign maneuver of abrupt changes. Fixed Signs dislike changes in routine (understatement) so they get it over with as fast as possible and settle into their new routine as if nothing ever happened. Another way to look at the Uranus/Sun intra-aspects is to note this is a relationship where they allow each other a lot of personal freedom too. Sometime so much freedom they can wander away from each other for periods of time.
Since they’ve been together for almost 20 years, I’d say they have discovered a rhythm to relating that works for them. Good Luck, Scorp, and here’s too many more years together!!