Developer only developed the software and he knows each 
flows of his codes and logics, some times lack of time or 
mood they may ignore the some mistakes and also due to dead 
lines they may not find such errors and also they will be 
confident on their code, so considering all these developer 
if he tests his own software also he will be thinking that 
his code is still working fine.

Because anybody who creates something would have an
emotional relationship to his creation, In this case a
developer would have an emotional relationship with the code
as he has written the code and would never like/want any one
(including him) to find bugs in his code.

The reasoning holds true in any job where the task(s)
performed is mission critical. Just as in accounting or even
in TV & papernews, you need a different set of people who
can validate that the solution, product or information is
correct.

its like why cant a student correct his own test papers!!!

there are two reasons 
1.a person cannot perform two tasks at a time
2.sentimental attachment