The two main factors that led to the outbreak of the French & Indian War were:
A) Economics - increasing the value of the natural resources and goods coming from the British colonies and French Canada. (Though it is worth noting that the populations of each were drastically different - American colonies numbered about 1 million persons, as opposed to the New France colony of roughly 60,000. The British colonies were much more profitable than the French colony.
B) Territorial expansion - the area in question was from present day Pittsburgh through to the Allegheny and Monongahela Rivers which eventually flow into the Mississippi River. Remember, the Mississippi River basically was controlled by the French. The American colonies were basically boxed in with the Atlantic Ocean on the east, the Mississippi River on the West, Spanish Florida in the South, and New France in the North. The area west of the Appalachian Mountains to the Mississippi River was primarily inhabited by native groups. The White traders/settlers began moving into this area in question and the French were alarmed that they might lose control of this so moved troops into the region. This eventually led to the Native groups siding with the French because they had been treated more fairly than by British/American settlers.