Why is the first step in responding to life-threatening situations to assess the situation?

Because you want to make sure everything is safe before you approach the situation. Another reason would be so you can see what is happening, and make clear, informed decisions based on that.
Another reason could be is that if/when you call Emergency Services, you can give a clear description of whats happening and where you are, number of people involved, etc. so they know whats happening faster.

I hoped this helped!
-Joanna