There really doesn't need to be a reason other than "because I want to", because the incremental impact of one car is low enough that no-one really thinks about

There really doesn't need to be a reason other than "because I want to", because the incremental impact of one car is low enough that no-one really thinks about it. If you think of people who drive cars as some sort of careless demons you've missed the point really - it's purely an economics problem. I own a car and have used it to drive in on occasion just because I can. It's fun to roll about the streets and do stuff like drive down Shaftesbury Avenue. This is precisely why taxation is necessary, because people don't realise that what they're doing is an issue in aggregate. If you had to pay the cost of a taxi to get to the local supermarket then people would figure out alternatives like cycling/taking the bus fairly quickly (and the supermarkets would adjust their business models, you'd see smaller ones more dispersed). I think part of the issue is that people can't seem to juggle the concept of something being fun and good but ultimately damaging. I might enjoy a pint every now and then, but I'm fairly happy about the fact that we tax it.