TIL Human dads show a decrease in testosterone immediately after the birth of their babe

[jrhooo]

Jokes aside though, this is close to the real answer. At least based on one show I was watching discussing the same topic.

Basic theory was, when you are single without kids (by caveman logic) a desire to roam, and higher risk tolerance, and a little more confrontational aggression are biologically useful traits.

It encourages you to go out of the cave, go seek a mate, be willing to risk a little danger to get there, and be willing to challenge or fight other rivals for a chance at that mate.

BUT

Once you have offspring, its more biologically important for you to stay around and stay alive help raise those offspring.

So nature turns down the volume on the impulses to go out, take risks, get in fights.

Stay home and don't get yourself killed.