Why was nationalism a big factor in world war 1

They were patriots, so everybody wanted to protect their country.

Ok look at Germany, for instance, they were very nationalistic. Think of
Kaiser Wilhelm ll, aka the king of Germany in the time of ww1. He is a man pure to the German soul, therefore, a patriotic. His strong beliefs is what led to the outbreak of ww1. but if you do further research, some historians argue that he was "controlled" by his generals. The argument is very controversial, but interesting.

Nationalism is dangerous because rulers tend to become greedy and resort to harming other countries in order to achieve their nationalistic goals. whether that be sustaining a better growth through trade. Or to control more lands, therefore expanding their empire and Increasing their power and control.

I hope this helps.