Because Boxer thinks he can out-do everyone when it comes to work. His personal motto is "I will work harder!"
The character of Boxer represents the mindless worker who is used up and spit out by those who take advantage of him. He is not all that smart and trusts completely in the pigs. (His other stock phrase is "Napoleon is always right.")
In the end, he works himself literally to death in that he is of no use to the pigs any longer and Napoleon send him to the glue factory. When Boxer finally realizes their treachery, it is too late.
The death of Boxer serves as a warning to those who have blind faith in their leaders.