It's hot.

I've always wondered why people need to point out the obvious.

Do we do it out of a need to belong?

Do we really think someone may have not noticed it?

Is it our way of expressing interest/discomfort without whining?

Are we idiots?

The first question, I came to the answer of: probably. We like to know that we're not the only ones who are effected by something. If it's hot, we like knowing that someone else is hot as well. Instead of asking them, we think it nicer to make a statement, and see if they agree or challenge.
I've heard that down south, saying "it's hot out" is usually met with an immediate challenge of "what you can't stand/don't like it here?"

As for the second question, I've always figured it's a way to make a generalized statement, that is pretty much easy to agree with. When we've run out of things to say, stating the obvious keeps our mouths working, or else are brains kick in.

Saying "it's too hot" can be taken as whining, and up here, we tend to hate whiners. There's an old saying here "if you don't like the weather in New Hampshire, wait a minute, it will change" and while it's funny, they made fun of it saying "If you don't like the weather in New Hampshire, go back where you came from."

I've always thought the second one was closer to our actual thoughts.

All in all, I'm far too hot to do anything right now but eat chips and watch Doctor Who.