Final answer:
The narrator enters the building in order to see the aftermath of a fire and experience the story of the fireman and his search dog hiding in the rubble.
Step-by-step explanation:
In the story "The Baddest Dog in Harlem" from 145th Street, the narrator enters the building because he is curious about what happened and wants to see the aftermath of the incident. The building is the scene of a fire, and the narrator wants to experience and understand the story of the fireman and his search dog hiding in the rubble. By entering the building, the narrator is able to witness the impact of the event and gain a deeper understanding of the story.