Andrew the robot no longer wants to be a servant. He's no ordinary robot and wants to be a human. After he purchases his freedom, he tries as much as possible to make his dream come true. But at every attempt he makes, he is rebuffed by society. No matter how hard he tries to talk, act, and dress like a human, he will never be accepted as one. This is a society that still looks upon robots as inferior to humans, and the abuse of robots is widespread.
So it's not surprising that Andrew chooses to undergo a risky operation that will effectively turn him into a human, albeit one that will live to the ripe old age of 200. Yet even after the operation, society still won't formally accept him as a human. It's only when Andrew goes before the World Legislature and declares that he's sacrificed his positron brain so that, just like a human brain, it will decline over time, that he's finally, and officially, declared a man.