Then how do you explain me still being madly in love with my wife after we have been together 12 years? We have had our ups and downs and it hasn't been easy but we have always worked it out in the end.
I believe that you need to try and make it fresh and new by trying new things and staying connected through the kids and each other but at the same time have your own time. With this as a healthy balance I think you can last the rest of your days happy and together.
I'm 37 and have had failed relationships before this though. I do truly believe that even now with all the distractions of modern day life people can stay together. Maybe I'm a romantic and I've not always been this way but as I've got older I've become more and more invested in my family and my wife emotionally, financially and physically.
I also think that the modern selfish way of living breeds some very weak people in regards to commitment and that kinda makes it easy for them to not bother working on a relationship. At the same time they might never know what a true connection is with your partner as that takes time, effort, humility and selflessness.
Humans are complex and are governed by more than simple socio-biology, as such no individual can be predictable. But the behaviour of general populations can be.
At the nasty end of Socio-biology you have the "Baby P" situation.
Where this happens it is almost always the case that the infant is abused to death by the male partner who is not the biological father while the biological mother does little to intervene. Just like Lions do when they take over a pride and kill all the cubs that are not theirs and the lionesses sit back and let them do it.
Some people are just more driven by their instincts than others.