What are the Causes / Reasons of software defects?

After building software sometimes software doesn't work as expected shows bugs , errors. Why is that software system doesn't work as expected ? There may several possibilities of thes

1. Someone used software incorrectly 
2. Someone makes error or mistake in software
3. Someone add invalid inputs in software
4. Incorrect development 
5. Issues in development 
6. Testing missed bugs.. 

There may be many more reasons why the software is not working as expected. It's depend on situation in which error or bug occurs. 

Do our mistakes matter to software? will they hamper ?

 Will discuss about effects of defect now. Its true that any in software development life cycle every entity human being like Business Analyst, Customer, Technical Architect, Programmers and Testers included, can make an error. These errors may produce defects in the software code or system, or in a document. If a defect in code is executed then the system may experience a failure. So the mistakes everyone in life cycle matter partly. It's severity depend on bug.

Our mistakes are also important because software systems and projects are complicated. Many interim and final products are built during a project, and people will almost certainly make mistakes and errors in all the activities of the build. Some of these are found and removed by the authors of the work, but it is difficult for people to find their own mistakes while building a product. And if bug occur in production then it may impact whole business.(Depend on severity).