Why do people prefer public transport over private transport?

I think it depends where you live. In Malta people prefer their own vehicle, although there is alot of traffic, but for the public transport it takes an hour or more for the next bus to arrive, and to go to a certain place you need to pick up several buses, so with your own vehicle, you go were you want and at what time you want.

I have a car. So I prefer private transport.

Well, in many places in India, and in some bigger cities, people who have substantial money go for Private over Public Transport as the Public transport, barring Metro (In Delhi and some other Indian cities) like buses are quite loaded, and there are safety concerns at times too, especially for the womenfolk. But yes, maintaining a Private vehicle, be it a car or a two-wheeler is quite expensive. So one goes for what on finds better !

Definitely prefer private, although I don't often have to take the bus or train (luckily)

Public transport can be more effective, cheaper and better for the environment

Since it is cheaper.