When I ask myself this question, the fist thing that pops into my head is that people don't think they can do something (or should I say make money from something) that brings them joy and happiness.
Unfortunately, although so many of us believe this, including myself sometimes, it is clearly a blatant lie. Somehow we grew up thinking that what we want to do is out of reach or not practical i.e. can't earn us a living.
If you want to be a professor for example, you might hear people talking about how it's so hard to become a tenure professor or that academic jobs are so competitive to get into. That there is little security in the job. The reasons go on and on. How do we know this is true? How do we know it would be true if we decide to follow that path? etc. etc.
But wait... aren't people already doing it?
The last question: Aren't people already doing it? Is the thirst-quencher. It's the knockout punch. No matter what you want to do, someone is already doing it and probably doing it very successfully. SO successfully that they have enough money that they wouldn't have to do it for the rest of their lives if they didn't want to but still they decided to do it. There are many successful professors who have earned tenure. Perhaps things are changing in the academic world but why wouldn't YOU be able to become a college professor? Why is anyone else better than you? Why would they be more capable of doing it than you?
They're not... The difference between you and them is that they did it despite all those other people telling them it couldn't happen. Or maybe they were lucky and found encouragement for their dreams. Either way, it doesn't matter. They did it and so can you and so can I.
Whenever we find ourselves doubting our dreams and our own abilities to achieve them, we must remember that other people already did it. The journey may not be easy and things are always changing so it might be different when you get there. We can't let that stop us. We must go after our passions and not give up because someone told us we couldn't do it. We must remember it's already been done and seek out opportunities to surround ourselves with people who are going to encourage us rather than encourage doubt.