Honestly it depends on the vehicle. I've named some cars and some bikes and others never even thought about it.....it depends on how much time I spend and how much I enjoy the machine. I don't go into it trying to think of a name when I buy something, it usually just happens because I "love" the vehicle and it's earned.....if that makes sense.
I bought a 2002 Trans Am WS6 brand new and spent a shit load of money, blood, sweat, and tears on it getting it up to close to 700hp and hooking. Many nights racing it, many days at the track. Had it for 8 years and named him Mr. Brownstone, because the damn thing was like heroin. Took all my money, could kill me at any minute but the thrill was so addicting. Names are earned I think, not just given out.