Because you won't remember the screaming or the crying, even though the day is probably full of them (from both of you). You won't remember how tired you were, or how many times you forgot to shower or wore sweatpants, or even the number of times you forgot to even get dressed. You'll forget about those long days of nursing, potty training, and scramming children in & out of the car while running errands and trying not to pull your hair out. I've said it before, the days are long, but the years are short. You'll probably try to tell yourself you'll remember, but you won't.
Sure, there will be times when you flash back to those early days of being a parent, scared shit-less, so tried you don't even know your own name, mourning your old self, both phsycially and mentally... but then you look down at your kid and smile. You can't help it, because they've done or said something so incredible or insane that all that nonsense above gets shoved way back in your brain and you melt like a Popsicle.
"I love you mama," "Mama you look so fancy", "Thanks for coming back mama, I missed you," "Mama, I love your dress," "Thank you mama," "Can I have a hug." (and so on... it kind of takes your breath away, and the moments when they scream at you to "go away" don't seem so bad).
Remember you'll cherish the chaos & weird photos just as much as the photos love and snuggles ones... so embrace all of it!
AND Remember, there are just some days where pants are really over rated...