How do you say no to any opportunity? I mean , as an engineering student, i have learned that anything can be made or any paper can be cleared if we set up our mind towards it( or if there is a deadline/good bucks/both attached to it)

But as a person who has given most of his free time to android dev, i know that i will give better outputs as an android dev than reading some web manuals for 2 days and work as a backend dev.
I am very confused. i have seen people who are very successful yet not passionate about any language , framework or tool. whatever comes in their way and carries a ton of $$$, they shut up, read the docs and make a great product. And then there are people who will only prefer to work in a specific environment and with a limited set of tools and technologies.

Can anyone with industry experience guide me that where should I incline if i am playing for the long run?