A favorite anything. Gun, knife, guitar, bike, car, whatever. What is that often indefinable something that makes it your favorite? And how would you describe it?
You know what I'm talking about. You pick something up and, wow. It's like it was made to fit you, or knows what you're going to do and helps. A gun that seems to steady itself as you aim; a knife that slips into your hand and cuts like it had its' own guidance system. A guitar that lets your hands slip along without effort. A car that makes it seem you can feel the road and what's going to happen as you drive.
And what makes that one different? You can pick up two of the same thing: same model, same everything, and while one feels like it was meant for you the other feels like junk. Maybe not junk, maybe just not as good in some way you cannot describe.
I've got a Tacoma guitar that I thought about for nearly a month, and then drove a hundred miles to buy. I just couldn't forget how it felt to play it, the sound it produced, and I wanted it. Still have it. Unless something happens that I just cannot play it anymore and I give it to one of my kids, I'll have it 'till I die.