Its not always difficult, Rider. Sometimes its wonderfully easy, with a solid marriage. Tho it takes effort to have such. Thought I should make this point since noone else has.
Again, it all goes back to that commitment. Plus the ability and desire to set up things that maximize everyone's happiness.
Marriage & family is really a multi-variable problem. Its difficult b/c most people can't even deal with a single variable in their lives (themselves). Throw another person, or more, into the mix and its chaos unless you have the qualities I mentioned. Trouble is, too many people think that the solution to their personal issues is another person, they don't understand they are increasing the complexity of their situation, not reducing it. Like DM said, being an idiot definitely doesn't help.
Hope that makes sense. That's really the answer as simply as I can explain it.