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It's not that unheard of. Basically, there's a big difference between knowing how to do something (or in Boris' case, maybe thinking he knows) and actually doing it. A lot of times people are full of advice about something, but when it comes to actually doing it themselves they're (pardon the pun with regard to Boris) just plain chicken.
For example, I've read in a survival handbook how to outwrestle an alligator. I have it all totally memorized. But if an alligator ever gets its teeth on my leg, you can bet your life I'll be scared silly.

You have a point, Jenna And Balto seem far to adventurous to be parents, especially Jenna.

Well, that's debatable. To begin with, it probably took Balto more guts to talk to Jenna about having a relationship than it did to face down that grizzly. Also, consider that they only left Nome because of extenuating circumstances, not any discernable wish to have an adventure. To say that they were adventurous simply because they went on an adventure out of neccessity... well, you might as well call Bilbo Baggins a great warrior.
Besides, parenthood can have its own challenges (I know this from watching, not actual firsthand knowledge). I think Ingred Cortez summed it up nicely in Spy Kids when she said in the story of "The Two Spies Who Fell in Love,"

And later she added,

So really, one might argue that Balto and Jenna were more adventurous as parents than otherwise.

ye

I think that everybody changes after beeing married and becoming into a father, maybe they change as anyone will do for maiking a nice relation, tou can´t understand it if you have not experinceof beeing it.

Exactly.