I think at a certain level we all know this, but in the same way we know "you lose weight by eating fewer calories." Easy understand, hard to live by.
In a way, it's amazing that 30% of marriages end up happy. It's hard to be kind to someone you don't respect, someone you think makes bad decisions, someone who isn't kind back, someone who is emotionally selfish or self-centered, etc. Take the large percentage of the population that fits into one of those categories, then square that, and you're well on your way to understanding the statistics.
Criticism is, in particular, a minefield. On one hand, once you're married, especially with a child, you have a shared future. What your spouse does or doesn't do affects you. On the other hand, it's impossible to deliver criticism in a way that doesn't tear at the relationship at least a little, and easy to deliver it in a way that tears at the relationship a lot. It can be hard to let go of the thought that: "if he/she just did X, Y would be better." But in many cases, fixing whatever our partner is doing wrong doesn't justify the stress of the criticism. As my wife says: "being right isn't a defense to the charge of being an asshole."