Pages: Why does pasting a title affect body text formatting?

I have to admit that working with text has always caused me problems when it comes to the technical aspects, although Pages is quite intuitive, but from time to time it still refuses to do what I tell it to do.

I’ll try to explain.

I have a text with a title formatted differently than the body text. Then I want to change the title. Maybe I copy-paste it from somewhere - and then: wham! The entire text is formatting like the title! I didn’t tell it to do that so why does it do that?

I try to reformat it to body text, but then the title is also formatted like body text.

I understand that they have become linked together somehow, but how? And why? A programme should do what I tell it to do, in my humble opinion, not messing with me like that.

But I’d be grateful if anyone can explain to me why this happens?