@sanjikun is saying it like a certainty that Loki will join Luffy because he wants to be with Nika.
However, I suggested that it's possible the context implies he isn't interested in having fun adventures like Luffy's crew is, he's actually interested in taking over the world like Xebec was and that's why he would be interested in Blackbeard instead. I am not saying this as a certainty, I only presented the possibility.
It's also a matter of personality difference. Loki is ruthlessly pragmatic. He prefers not to be concerned with others. Since he was young, the world around him hated him, and when he saw Xebec, he made "world domination" goals.
Can you see Loki chilling in a giant geyser with the Straw Hats for adventure? I am not sure I can. He might disagree with a lot of things. For example, instead of adventuring, he would want to be waging war. His entire personality would have to change for him to be socially acceptable among the Straw Hats. When you think of it culturally, he doesn't really share their culture.
Currently, if a situation came up where the crew asked Luffy what he intends to do, and Loki suggests they go to war, but the crew addresses a concern that many people could die in it, Loki would simply respond "Why should I care about that?"
That sort of thinking aligns with Blackbeard's crew more than it does Luffy's crew. Let's bear in mind that Loki is the embodiment of a War God right now. A War God's culture might not fit with the Straw Hats.