Why we use design patterns ??

Well, design patterns provide the programmer with a standard 'object oriented' way of dealing with a particular problem. As such they are a way of reusing code so you don't have to write it from scratch each time a particular problem is encountered.

Having said that, I can't say I've ever been a major user of design patterns.

That's because:

1. The approach used by the design pattern is not necessarily the same as I would have used myself.

2. The situations you encounter in practice always seem to differ somewhat from the situation which the pattern is designed to solve and the latter may therefore need some major modifications to solve the problem in hand.

For more on design patterns, check this link:

http://en.wikipedia.org/wiki/Software_design_pattern