I think this explains it pretty well...
The Special Mother
by Erma Bombeck
Most women become mothers by accident, some by choice,
a few by social pressure and a couple by habit.
This year nearly 100,000 women will become mothers of handicapped children.
Did you ever wonder how these mothers are chosen?
Somehow I visualize God hovering over Earth
Selecting his instruments for propagation with great care and deliberation.
As he observes, he instructs his angels to take notes in a giant ledger.
"Armstrong, Beth, son. Patron Saint, Matthew."
"Forrest, Marjorie, daughter. Patron Saint, Celia."
"Rutledge, Carrie, twins. Patron Saint...give her Gerard. He's used to profanity."
Finally he passes a name to an angel and smiles. "Give her a handicapped child."
The angel is curious. "Why this one, God? She's so happy."
"Exactly," smiles God. "Could I give a handicapped child a mother who knows no laughter?
That would be cruel."
"But does she have the patience?" asks the angel.
"I don't want her to have too much patience, or she'll drown in a sea of self-pity and despair.
Once the shock and resentment wear off she'll handle it."
"I watched her today.
She has that sense of self and independence so rare and so necessary in a mother.
You see, the child I'm going to give her has a world of it's own.
She has to make it live in her world, and that's not going to be easy."
"But Lord, I don't think she even believes in you."
God smiles. "No matter, I can fix that. This one is perfect. She has just enough selfishness."
The angel gasps, "Selfishness? Is that a virtue?"
God nods. "If she can't separate herself from the child occasionally, she will never survive. Yes, here is a woman whom I will bless with a child less than perfect.
She doesn't know it yet, but she is to be envied.
She will never take for granted a spoken word.
She will never consider a step ordinary.
When her child says momma for the first time, she will be witness to a miracle and know it.
I will permit her to see clearly the things I see--ignorance, cruelty,
prejudice--and allow her to rise above them.
She will never be alone.
I will be at her side every minute of every day of her life
Because she is doing my work as surely as she is here by my side."
"And what about her Patron Saint?" asks the angel, his pen poised in the air. God smiles.
"A mirror will suffice."
I'll never forget the time I had a full weekend off and decided to make a big old fashioned Sunday dinner for the four of us. I made everything homemade, set the table, etc. and as we were eating my son leaned over and whispered, "Mom, do you know there are families that do this every night...
" 
exactly!