Why did the building of french fortifications alarm the governor of virginia?

The governor of Virginia was ordered by the king to spread into the Ohio valley river because of the rising population and the need of expansion. The french built forts so as to scare people away and prepare for a possible war, which was alarming to the governor of Virginia who couldn't spread into that land because of them. When he asked the French to leave it, they denied him his request and soon after the war began.