Why does Sharpless warn Pinkerton about marrying Cio-Cio-San?

Sharpless, the American consul, warns B.F. Pinkerton against his marriage to Cio-Cio-San because he recognizes that the young woman is deeply and sincerely in love, and he fears that Pinkerton’s casual, transient approach to the relationship will inevitably break her heart.1

Sharpless observes that Pinkerton’s lifestyle is shallow and cautions him that his behavior could lead to sadness. Furthermore, Sharpless explicitly warns Pinkerton that the marriage is not a joke because Butterfly truly believes in the commitment she is making. Despite these warnings, Pinkerton remains dismissive, viewing the marriage as a temporary arrangement and treating his bride as a "toy" or a "butterfly" to be captured, even if it means causing her harm.1