Good one Virgo Burger! Maybe they managed to balance all those differences out because they are both mature and forgiving? My parents also lived together untill they died. I always thought they were happy until they died and I found letters. It was not all too great, I can tell you that.
I myself was married for 34 years and then: divorce! Everybody thought we were the ideal pair. Our synastry only gives positive aspect, honestly, you wont believe it, but we were together in this life to learn from one another. There was affinity but both were not really happy,certainly not him. I was content but never got the love and closeness I yearned for. We did not act like a couple, everything was done separately. Now how is that possible with such a fantastic synastry? We are still very good friends though so maybe that's what the synastry was all about.
Personally I dont like synastries. I rather look at the two charts and see how these people are and if that would be OK together. dont forget also that many so called stress aspects, show dynamic interaction, not awful fights. Sun-Moon oppositions, very popular in good marriages for instance.
The Venusses of your parents (rulers of both their Suns!) are traditionally both ruled by Mars and their Moons both by Saturn (I find traditional astrology very informative even though I was taught modern). they also both have a Mercury-Saturn aspect (which can be brought into connection with the rulers of their Moons because of Saturn). One the opposition, the other the conjunction, so they both think and feel in a similar way, the same insecurities in connection with their way of communication and feel therefore safe with one another. This I think is one of the major reasons. No matter what, they knew they could count on one another.
By the way, I will move this thread to the Greenhorns Lounge as you did not give your own ideas about the "why is this"