Why do you think the lawyer extends the bet from 5 to 15 years?

Initially, the banker makes the bet for 5 years. He believes that the lawyer would not be able to endure 5 years of voluntary imprisonment. In the heat of the moment, the lawyer raises the stakes to fifteen years. He probably does this to prove how serious he is, and how much he believes that he is in the right. This is corroborated by the fact that he ultimately intentionally loses the bet just 5 hours before he would have won it: he makes deliberate choices in an effort to prove his commitment to his principles.