So our first human may have said to his newly minted mate, thus uttering the world's first palindromic phrase.

But speaking of Adam, I think what Milton does in fleshing him out is interesting and significant. When I think about what he needed to do with this character, I stop at piety and obedience toward God. Maybe another needed element was for Adam to show too much love for Eve, following the tradition that Adam commits the offense of "uxoriousness." But Milton also makes Adam comtemplative, philosophical to a degree, and, as we see later, inquiring. Why does Milton cast him this way? Why not just make him just a happy, simple man who asks no questions? Isn't that what God wants? (Note that our first view of the couple is through the eyes of Satan, whose discernment adds to his depth of character.)

For contemplation hee and valour formd,
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His fair large Front and Eye sublime declar'd [ 300 ]
Absolute rule; and Hyacinthin Locks
Round from his parted forelock manly hung
Clustring, but not beneath his shoulders broad:

Anyway, that's my question: Why does Milton make Adam deep?

Free will. It only works if one can think about options and chose. I think it is in book V or maybe VI that Milton states clearly that God does not want blind obedience, for there is no value in obedience and loyalty without the choice to do otherwise. I will find the line numbers and post them in later today.

Maybe Adam is Milton's representation of himself.

That could be, though I see Milton pouring more of himself into the angel Abdiel. It seems sure that Milton couldn't envision a human (male, anyway) without the ability to reason and thus to speculate. That makes sense if the human (male, anyway) is made in God's image. What about dramatically in the poem? There seems to be a function there, too.

Well, there'd be no story if Adam simply followed the rules (not to mention no world as we know it). Even after Eve has eaten the forbidden fruit, Adam still has a chance to say no. It is because he can reason and speculate on how it would be in the garden without Eve that he chooses to eat the fruit.