Software has bugs because
Bugs get introduced right from the requirement analysis 
phase
--The customer requirements are not properly understood
--The design phase gets affected by the above
--The coding based on the above faulty design introduces 
bug in the software

If the developer follows all the required  procedures for 
designing and coding which is based on SRS and follows all 
the required procedures like using decision tables,etc. 
then the software will not have bugs at all. This is an 
ideal situation. But practically this is not possible. The 
coder commits a small mistake somewhere in coding or 
referring the table,etc. Though it is small one, it will 
have huge impact if released without proper verification 
and rectification, like Spacecraft/ satellite failures  
soon after launching or in the mid way. This comes from a 
famous saying "To err is human".

i think due to reason software has bugs 
miscommunication or no communication
software complexity-diff comp environments
programming errors-changing requirements
time pressure- due to lack of time,dead lines
poorly document code

1)miscommunication or no communication - as to specifics of
what an application should or shouldn't do (the
application's requirements). 

2)software complexity - the complexity of current software
applications can be difficult to comprehend for anyone
without experience in modern-day software development. 
programming errors - programmers, like anyone else, can make
mistakes.
 
3)changing requirements - the customer may not understand
the effects of changes, or may understand and request them
anyway 

3)time pressures - scheduling of software projects is
difficult at best, often requiring a lot of guesswork. When
deadlines loom and the crunch comes, mistakes will be made. 

4)poorly documented code - it's tough to maintain and modify
code that is badly written or poorly documented; the result
is bugs.

5)software development tools - visual tools, class
libraries, compilers, scripting tools, etc. often introduce
their own bugs or are poorly documented, resulting in added
bugs. 

prabha.bhatted@hotmail.com