I was about to say that I can't explain it...but I'll give it shot:
For me, that old GM stuff (and the British stuff) is sorta like the girl the grew up across the street from you. You know everything about her; her faults, her alcoholic mom, her scars, fears and weaknesses. But you also know all the good things about her too; all her strengths, how to care for her, her likes, dislikes, etc. Imagine the bond you might share with someone you know that well. Any "other" car is more like the girl from a few towns over. Sure, maybe she's gorgeous, trendy, well dressed, and maybe you've taken a test drive or two; but you only know what you've heard. Now maybe the mystery is intriguing, maybe the grass is greener, but will you ever be able to share a bond comparable to what you could with the first girl?