Shiro was walking around a planet that had recently used their destress signal. When he got off his Black lion all he saw was… destroyed Galra cruisers, fighters, sentries, and Galra etc. And in the mist of that destruction was you destroying the last sentry. When you were done you collapsed. Did you do all of this..? Did you save the planet all by yourself?

He decided to observe you before talking to you, to possibly get some information about what happened.