China has been building up its navy for a while now. It has no choice. During the last confrontation with Taiwan the US sent 2 carrier fleets to make sure China didn't fuck around. Before China can take back Taiwan it needs to beef up its navy, which it has been doing. A map of Taiwan Strait will make things easier to understand:
If China can't control the Taiwan Strait then it can't control anything. It is only logical that they are going to do everything in their power to regain influence over their own back yard. And you can be sure that China will eventually act to take back Taiwan one way or another.
For example, recently declassified documents show that in 1969, while everybody was watching the war in Vietnam, the Chinese and Soviets were engaged in a full scale battle with 1 million Chinese troops fighting against around 750,000 Soviet troops. This battle started because the Soviets controlled territory that the Chinese believed belonged to them. The Chinese started the fighting by opening fire on and killing Russian troops patrolling the disputed territory. When all was said and done and the fighting was over China kept the territory they wanted.