I always thought kerbals produced asexually. Like mushrooms. And they have a short life span, mainly because they hold their Farts and Waste in until they reach 10 and then explode. And they fly rockets to try and get the maximum thrill before they die.
Science stuff, Damaged parts that can be repaired with kerbal duct tape. And fire/Craters when you crash your rocket. Maybe resource leak too, like if you puncture a fuel tank, it will slowly leak until it is fixed.