why should the american colonists fight, what is their purpose ?

The American colonists fought the British for one main reason.  They fought because they wanted to be independent.  Since you ask for two reasons, I assume that you need two reasons why they wanted to be independent.

One reason the American colonists wanted to be independent was British taxation.  Many Americans did not think that the British government had the right to tax them.  

it was on the rise and doing very well and would continue to do well until the great depression from 1929-1939