The process of birth, with all its uncleanness, is itself considered a crisis, possibly the first in a long series of “severities” (Gevurot) to be experienced.
It is in the nature of the fetus not to continue its fantastic growth in the womb but to be cast forth into the world.
The child’s growth then has to continue in a different framework.
He has to pass through the ordeal of weaning, which is the cessation of being fed, and he has to keep moving on to phase after phase.
If a child ever gets stuck at any particular stage, he becomes something else; there can be no lingering.
–Rabbi Adin Steinsaltz