There is a common advice - if you have a dream (by dream, I mean goals or ambitions), keep it to yourself. Don't go about telling people.
This is because of various reasons. Some people believe that when you tell others about your dream, you somehow jinx it. To others, telling people about your dreams, could draw their envy and resentment. The reason that inspired this insight, however, is that of discouragement.
Some people tend to discourage others from the pursuit of their ambitions. This, I have experienced. The question however is why would someone discourage others from going after a purpose they have set to achieve?
Above anything else, the most common answer is because they themselves have failed in a similar pursuit. If a dream becomes unachievable for a person, he adopts a cynical view to it. Believing that it is also unachievable by others.
As a budding filmmaker, one of my goals is to cause a positive change in the Nigerian film industry. An industry, begging for change and restructuring. However, in a lot of the filmmaking conferences I attend, there are guest speakers who start off by saying 'don't think you would change the industry, I used to think so too'. Judging from reactions, that statement stings a lot of attendees.
I realized that these speakers make this statement, because they failed to achieve what they wanted to - effect a change in the industry. Along the line, they let the frustrations of the industry get to them. And they compromised standards. So what they have decided to do is to poison the fountain, if they cannot drink from it. Misery after all, loves company.
This is also applicable to other professions and industries. A lot of people do not want to see others attain that which they couldn't. Some genuinely believe it's unattainable, others just cannot bear to see others succeed where they have failed.
And the truth is this, amongst all other reasons you may be discouraged from pursuing that which you dream of, this is most effective. It's easier to dismiss the envious man, but harder to ignore the one who claims to have towed that path you're own. It's easier for doubts to creep in, when you see a man who has failed in the same quest that you're on. There are a lot of ambitions which were let to die, because of the pessimism of others.
But there's a shield against this. That shield is to believe rather than to merely think. Do not just think you'd make the next ground breaking discovery in medicine, believe it. Too many people misunderstand these two things. Thinking is what gets you started. Believing is what keeps you going. To shake off the cynics and begrudged of this world, you'd need a healthy dose of believe.