3. Lucifer was always fated to fall in to temptation
It's a cruel irony, but Lucifer has always been fated to fall into temptation. Even before the creation of humans and his fall from grace he'd always been one to push the envelope. It's probably why he's the one to make the first move.
He knows he can't be the only one to feel the connection, but apparently he's the only one willing to do something about that. Michael is hesitant, but Lucifer is persistent. He has time; he'll wait for Michael if he has to.
They start off slow, a few shared looks across the room, a light brush of fingers, or time spent under the stars lying next to one another. Lucifer is patient; he knows he can't rush Michael into anything. It pays off in the long run.
Their sitting under the Tree of Knowledge looking out on the rest of the Garden of Eden, when Michael reaches for Lucifer's hand winding their fingers together.
Lucifer holds his breath at the contact, it's the first time Michael has reached out first. Hesitantly he looks to his older brother who simply gives a small nod a shy smile on his face. Lucifer smiles before leaning forward lightly kissing Michael.
There's a definite spark and Lucifer is happy when Michael starts to move with the kiss. It's both the best moment of Lucifer's life and the start to his downfall. Of course he doesn't know that, he's focused on deepening the kiss, happiness flowing through him as he realizes that he finally has Michael.
Only angels weren't ever really meant to partake in such human aspects of life, and when Michael wants to end it, Lucifer just can't. All he's ever wanted was this, and now Michael is pulling back from him, withdrawing completely, his eyes moist with unshed tears as he shakes his head. They can't do this anymore.
A part of Lucifer knows that Michael is right, that this was all wrong from the start, but mostly he focuses on how unfair this whole thing truly is.
He grows bitter, and it only gets worse when Dad creates those mud monkeys. Falling comes easy after that. He doesn't have anything to stay around for and if he leaves he can go off his own rules. That's the most appealing thing he's heard in a long time.
He's known from the start that he was different. That he wanted things he wasn't supposed to have, only now he doesn't really care. What's truly stopping him from having those things? Surely not Michael, and so with the promise of freedom he leaves, falling from grace never looking back.