Answer:
Germany was lagging behind many countries, mostly Britain in the arms build-up. Germany wanted to grow out their militarism to either keep its nation safe or to expand, all for its self-benefit. It was also to show that Germany was capable of independence from the network of allies they had.
(Counter Arguement below)
However, since one country had joined an alliance with others, their military powers would be combined and would've had a bigger impact than a single country. The network of alliances let nations help each other in war and with this extra power, they were able to control and have more power against a single nation.