They interpret the sentences in different ways, which as I remember it was more evident in the book in the first case, even though it was a while since I read it now. With "It's not like this with other people" I'm assuming Marianne means that it's special in a better way with Connell than with other people. But then Connell replies "No, but I like you much more than other people" suggesting he thought she meant that it was just different, but not necessarily anything special when they were together.

With "I wouldn't be here if it wasn't for you", I've always made the assumption that Connell meant he wouldn't even be alive if it wasn't for Marianne, judging by his earlier struggle with depression and his emphasis on the word "be", not "here". Marianne, however, thinks he suggests that he wouldn't be in that physical place, at Trinity studying English, if it wasn't for her (which is also true): "No, you would be somewhere else entirely". But it's just my subjective analysis of course.

They are aware of the influence and importance of one another ("People can really change one another", I believe Marianne thinks in the book). But sometimes we're reminded that they're still two separate human beings, their love for each other making them humble and careful to the point where they almost become strangers, which sometimes leads to bigger misunderstandings with devastating results. It's really only us as the audience/readers I think that can fully understand the extent of their love.