Question: What is the difference caused by using “nope” instead of ”no”?

Is it used because “nope” sounds better and not straight like “no”? In some situations, it feels like *nope* is better to use than *no* even though it adds 2 more characters. What is the reason why many use it?

Answer: 
The first and most important point to note it that it's *very* informal (more so than using contractions such as my ***it's*** there, for example).

The main reason for using it at all stems from that "extreme informality". It normally conveys a *relaxed attitude* on the part of the speaker. Depending on context, it can be more *or* less emphatic than "No".

You only use *nope* to mean ***[my answer is] "No"*** – it never replaces *no* in any other contexts. And you wouldn't normally use it where you want to be *very* emphatic (shouting "No!" at the top of your voice). Which example illustrates a defining characteristic – "Nope" isn't often followed by an exclamation mark!

Finally, I'd echo [John Lawler's](https://english.stackexchange.com/a/59670/2637) words: *"nope" occurs **only as a one-word answer** to Y/N questions*. That's to say, a written form such as:

> 
> "Nope I *don't* want to"
> 
> 
> 

doesn't look right. We expect a full stop (or at the very least a comma) after "Nope" because in real-world speech there always would be a pause there. But that doesn't happen with:

> 
> "No I *don't* want to!"
> 
> 
> 

because it's perfectly possible to speak those words without pausing appreciably after "No" (without necessarily placing extra stress on *"don't"*, either; I just italicised it as one possible enunciation).
I think the context has a lot to do with how the word Nope is perceived:

1. Somebody asks you for something.

> 
> - Will you do something for me?  
> - Nope. << Rude
> 
> 
>
2. Somebody asks for information.

> 
> - Is your name Sam?  
> - Nope. << Playful / informal
> 
> 
>

Maybe the word Nope implies a casual, relaxed attitude, which in some cases may come off as dismissive.