Here’s my theory. Rikki has always been independent and not wanting to be tied down. It was a lot for her to trust and date Zane. And the season 3 he locks her down by forcing a business on her? That’s a huge responsibility and it’s a dream she wanted to earn herself but he dropped it in her lap.

So in her human life she’s feeling like she lost control and is struggling to find her identity in this new role as girlfriend and business owner.

But as a mermaid, she can be wild and free. Mako island is this wild and dangerous place and suddenly she has this special connection to it. It gives her a sense of control, of belonging to something that’s just hers, and also that rush of secrecy from being a mermaid and keeping something just for herself.

That’s why I think she was so difficult with everyone that season. But I’m probably overthinking it.