The question was 'why' and the text didn't say. Since the text doesn't go into an grand exposition we are left to use our intellect to fully understand things.
The Adam did it for love narrative is something the church (everyone I've been to) has tried shoved down my throat. It's the root of the Disney fairytale theme, your not supose to let anything come between you and your one true love.
Here's my understanding.
When the woman saw that the tree was good for food, that it had a pleasing appearance and that the tree was desirable for making one wise, she took some of its fruit and ate. She also gave some to her husband, who was with her; and he ate.
[Adam was did not use his G-d given authority to stop her. I reject the churches explanation that she later found Adam and shared it. That he wasn't actually there. That's not what is said.]
Then the eyes of both of them were opened, and they realized that they were naked. So they sewed fig leaves together to make themselves loincloths.
They heard the voice of ADONAI, G-d, walking in the garden at the time of the evening breeze, so the man and his wife hid themselves from the presence of ADONAI, G-d, among the trees in the garden.
ADONAI, G-d, called to the man, "Where are you?"
He answered, "I heard your voice in the garden, and I was afraid, because I was naked, so I hid myself."
He said, "Who told you that you were naked? Have you eaten from the tree from which I ordered you not to eat?"
The man replied, "The woman you gave [to be] with me - she gave me fruit from the tree, and I ate."
[Refused to be accountable for the Authority G-d gave him]
ADONAI, G-d, said to the woman, "What is this you have done?" The woman answered, "The serpent tricked me, so I ate."
ADONAI, G-d, said to the serpent, "Because you have done this, you are cursed more than all livestock and wild animals. You will crawl on your belly and eat dust as long as you live.
[The serpent was still with them]
I will put animosity between you and the woman, and between your descendant and her descendant; he will bruise your head, and you will bruise his heel."
[punishment for his deception]
To the woman he said, "I will greatly increase your pain in childbirth. You will bring forth children in pain. Your desire will be toward your husband, but he will rule over you."
[punishment for convincing Adam to eat]
To Adam he said, "
Because you listened to what your wife said and ate from the tree about which I gave you the order, 'You are not to eat from it,' the ground is cursed on your account; you will work hard to eat from it as long as you live.
[punishment for refusing to use his G-d given Authority]
They both faced
death and seperation from G-d as their punishment for disobeying Him but ultimately Adam was held responsible because he refused to use the Authority given to him by G-d and srop her.
Where your see it the he did it because "
he couldn't bear to be without Eve; choosing to die with her rather than live apart." I see it as Adam refusing to step up and be accountable, to lead.
That's a theme repeated over and over. Man refuses to use the Authority G-d gave him for many reasons. The men who come to their senses and don't reject the Authority G-d gave them become Heros, the ones that do become villians.
Look at how G-d had to Choose Deborah because "
all the men of Israel did what was right in their own eyes". She had to convince Barak to lead, he did step up and lead and thats why he is lauded in Hebrews 11. The men who are praised in scripture are praised when the exerted the Authority given to them by G-d and obeyed His will.
Hebrews 11:32
And what more shall I say? For time would fail me if I tell of Gideon, Barak, Samson, Jephthah, also of David and Samuel and the prophets. By faith they conquered kingdoms, administered justice, obtained promises, shut the mouths of lions, quenched the power of fire, escaped the edge of the sword, were made strong out of weakness, became mighty in war, and made foreign armies flee.
The second Adam did what the first did not. He wielded G-ds Authority and was Himself accountable for the sins of others. I know it makes alot of men uncomfortable with the fact that no matter what reason we give or what we point our finger at and say but L-rd it was _______ fault, we are still the ones accountable for not exerting our G-d given Authority.