[wwweston]

10 and 14 kids are outlier numbers by late 20th / 21st century Mormon standards. 4-6 is more common and 2-3 aren't uncommon.

There's probably a bit of ag heritage at work here (though the mountain west has definitely been leaving that behind for the last few decades).

The religious narrative is that it's (a) it's part of God's plan for your personal development & happiness and (b) it's a duty to give a good spiritual home with a father and mother part of the true faith to as many of God's children as possible.

It's probably objectively common (though hardly universal) that people report parenthood to be a crucible of personal growth and a source of meaning and satisfaction, and it's certainly arguable it's evolutionarily adaptive to be that way, so (a) checks out pretty well (though it's a little less clear how well particulars like "marry at 21" serve people).

(b) is as unfalsifiable as any faith, but much in the way one can say a faith discouraging reproduction is likely to have a lifespan close to a single generation, faiths that encourage it are likely to have better internal replacement and growth rates, which might be especially important if you're a minority faith with modest conversion success.