I try not to over analyze things but I usually do. This is what happens when I'm sick and have way too much time on my hands. This is just my little take on why Dean loves the car so much.
The familiar rumble of the Impala was a welcome change from the pieces of junk they had been driving lately. It was more than okay with Sam that Dean had his "baby" back. He just loved to tease him about it.
"You know, if you two want to get a room, just let me know Dean," Sam smirked as Dean held fast to the steering wheel of his beloved car.
"Aw, don't listen to him baby," Dean directed his comment to the car, "He just doesn't understand us!"
Sam shook his head and smiled sadly. He understood perfectly, probably more than Dean understood himself.
That black beauty, before it was Dean's, was John's love. When Dean was old enough to drive, John gave the car that he loved to his pride and joy; Dean.
Dean respected that car. He kept it up, cleaned it, cared for it like is was actually alive... talked to it. No matter how creepy an obsession it was, Sam could see why it was Dean's obsession.
Dean saw his father in that car. The Impala was Dean's way of keeping John alive, if not physically, spiritually. John's presence while cruising down the road in the Metallicar would never go unnoticed, not ever again.
Dean would never let anything happen to that car, therefore, he would never let anything happen to John's memory. John lived on through Dean, and Sam knew that.
Yeah I know that was really short. But it was really just a drabble. Tell me how you liked it.