"I wasted my best years on you"

This was an interesting scene near the beginning, because you're seeing the friction Kate's having with still being forced to interact with her ex-boyfriend. You start to see why he broke up with her, because she was a loud, annoying bitch. That's why it's funny to the audience when he replies "Those were your best?"

However, when I was reading up on the behind-the-scenes for this part specifically, it shows a really big disconnect the director had between how he viewed Kate, versus the audience. See, he had her say that line to try and draw up sympathy for her from the audience, whereas the audience didn't understand why the time-travel guy even put up with her to start with.

To be fair, he's a socially awkward guy who's not always good at human interaction when confronted directly. Small wonder he got a girlfriend at all, much less someone like Kate.

We don't find out until later that she behaves the way she does because she had to work in a competitive market for advertising, she's had to go at it alone for many years and form a thick, tough, outer shell to protect herself. Sad part is, she didn't seem aware how it affected people until she got some perspective from Leopold. It's only after she interacts with him, letting him in a little bit, that we get to see her better side.