The man stared at Bepin Babu in disbelief because Bepin Babu did not recognize him even though he was sure that he knew him and had met him in Ranchi.

Introduction:
In the short story "Bepin Babu and the Man Who Stared," written by Satyajit Ray, the protagonist, Bepin Babu, encounters a man who stares at him in disbelief. This incident takes place after Bepin Babu loses his memory due to a head injury and tries to piece together his forgotten past. The man's reaction is a significant turning point in the story and adds intrigue to Bepin Babu's quest for the truth.

The Man's Disbelief:
The man's disbelief can be attributed to two primary reasons:

1. Bepin Babu's Forgotten Past:
When Bepin Babu first meets the man, he is unaware that he has lost his memory. As Bepin Babu starts talking about his past, mentioning names, places, and incidents, the man realizes that Bepin Babu has forgotten important details about his own life. This lack of recollection surprises and confuses the man, leading to his disbelief.

2. Bepin Babu's Accurate Predictions:
As the conversation progresses, Bepin Babu astounds the man with his accurate predictions about the future. Bepin Babu mentions specific events that later come true, such as the sudden death of the man's neighbor. This ability to foresee future events leaves the man in awe and disbelief, questioning how Bepin Babu could know such details without any prior knowledge.

The Impact:
The man's stare of disbelief has a profound impact on Bepin Babu. It triggers a realization within him that there is something more to his forgotten past. Bepin Babu becomes determined to uncover the truth behind his memory loss and regain his lost identity.

Conclusion:
The man's stare of disbelief serves as a pivotal moment in the story, highlighting Bepin Babu's forgotten past and his extraordinary ability to predict the future. This encounter propels Bepin Babu's quest to rediscover his identity, leading to further intriguing events in the narrative.