In html when you're closing a line of code, why must i put a slash mark?

Question

For example:

<p>happy</p>

Answer 1

Just saying that the actual anchor <a> tag is used for hyperlinks and you generally don't nest other <a> tags inside it. I think Lexfuturorum used <a> as a general tag name.

Answer 2

Because if you wouldn't you couldn't do things like
<a color="#ff0000" >
Red
<a color="#000000">
White
</a>
Red
</a>