Usually a perceived need or want - it's sometimes hard to tell the difference!
For example, last week I started a new job with a 3-mile commute. I'm driving a full-size pickup and that is a terrible abuse of it since the engine/transmission never gets warmed up and that will kill it in a few years.
If the roads weren't so narrow, windy, hilly and high speed I'd consider a bicycle, but doing that would leave me with a life expectancy of about 20 minutes, so that's out. So I went looking at motorcycles, thinking of something in the 250 cc range or even a 150 cc scooter, which I test drove. That doesn't have enough acceleration or top end speed for a 2-mile stretch of road so that won't work. The $1,500 price makes it attractive, but it's Chinese and the warranty is for one year on engine and transmission only. Not good. Lately I have been underwhelmed at the quality of Chinese goods so I'll pass on that.
Next I looked at a 650cc Suzuki (the smallest that shop had) but at $4,300 it was a bit much, so I next considered a 250 cc Honda Rebel at $3,200. I am sorely tempted but have learned in the past not to make snap decisions when I am "sorely tempted". Craigslist doesn't have much in the way of small commuter motorcycles either - everyone who has one apparently wants to keep it.
However winter and rainy weather will inevitably come, so practicality dictates that I'll probably end up with a used car for about the same amount or even less. If I buy a car that runs for $2-3k and it runs for five years I'll have gotten my nickel's worth.
But a motorcycle sure would be a lot more fun in the nice weather....