Hancock was a merchant and banker in the city of Boston. The two occupations were very lucrative when it came to making money and once the British taxes were imposed, Hancock was not a happy person. John Hancock and Samuel Adams were the most outspoken against Britain's policies and were heavily involved in the infamous Sons of Liberty. They were such a "thorn in the side" of the British that it was they who British troops were sent to arrest that led to the battles of Lexington and Concord in 1775.
The influence of Hancock and his wealth during the American Revolution led to him being elected president of the Continental Congress in Philadelphia. This would be the same Continental Congress that approved the Declaration of Independence. With Hancock being the president, he was entitled to sign the Declaration of Independence first. The size of his signature on the Declaration is a testament to his dedication to both American liberty and to himself.
Source of information: The Intellectual Devotional: American History by Kidder and Oppenheim, pg. 50