Ya, it happens in real life. But it doesn't make for a healthy longterm relationship. That's what makes it so frustrating that this appears to be the only relationship Mookie thinks exists.
This is the part that bothers me the most (yes, even more than the crappy plot and other problems). Mookie keeps writing characters that are in co-dependent relationships and makes it seem like that is healthy. Worse, each character picks up new magical powers during the relationship, which makes the entire thing even more creepy.

The best example is Luna and Dominic (who is the ultimate narcissist), but the Nimmel-Katya dynamic is even weirder. Nimmel was a bully in their world, and he is preying on someone who literally was an outcast with unrequited love for Milov. Seriously, how can he not see how screwed up that is?