When we are children we often talk about what our dream job will be. This is often a doctor, Vet, police man / women or actor / singer but as we mature a lot of us change our mind. So why do we change our idea of a dream job? This is usually because we start to think realistically about the job and what it entails or what qualifications need to be obtained to apply for such positions.
A Doctor for example can spend 7 years or so studying before they actually get a job and with this comes a huge cost at university plus not being able to have a full time job in that time. For some this is simply out of reach and therefore they decide to scale their ideas down a bit.
You should never give up on your dreams if it is something you really want and often there is a way to do it, but it can seem impossible with the day to day running of your life.