Question: I must admit, I entirely did not understand why

> 
>  Queenie 
> 
> 
> 

chose to join Grindelwald at the climax of *The Crimes of Grindelwald.* She seems like the least likely, given her relationship with 

> 
>  Jacob.
> 
> 
> 

Can anyone explain?

Answer: 
**SPOILER ALERT**

An obvious reason (properly described by @Valorum) is being able to marry Jacob. It's her greatest desire, and she sees Grindelwald's way is the only way to achieve that. It's like the last resort for her: either that or nothing. She doesn't seem to think deeply about the possible consequences. 

But there is more to it.

**She really shares Grindelwald's views, though in a more naive way.**

Although Queenie looks like a nice person, she already showed that the idea of using magic over muggles "for greater good" is fine with her. She put a spell on Jacob to make him stay with her, which is a pure violation of an individual's free will, but she thought she had the right to decide what is best, exactly because she had the power to benefit them both (as she saw it). Apparently using such methods on a no-maj seems legit to her, though she would probably think twice if Jacob was a wizard himself.

She also doesn't ever seem to think she should limit her mind reading to respect others' privacy. It's quite straightforward for her: you've got the power, you use it. If you are a good person, then everybody will just benefit from it, right?

Queenie is shown as a sweet, but rather naive person who does not think deeply about a problem, which makes her see right and wrong in a very narrow way. That's also the reason she can be easily persuaded. It didn't seem to take much effort from Newt to persuade her she shouldn't have put the spell on Jacob; in the same way, it was easy enough for Grindelwald to persuade her into his ideas.

So it is possible Queenie genuinely shares Grindelwald's views that wizards are allowed to use their powers if this is for "greater good."
Because she heard what she wanted to hear
-----------------------------------------

***To avoid a spoiler-formatting "hell", this post contains unprotected spoiler information***

In *Crimes of Grindelwald*

> 
>  Queenie
> 
> 
> 

join forces with Grindelwald at the end.

As others have suggested before, Queenie was in love with a Non-Wizard and by the standards of the late 20's Wizarding Society, marrying between Muggles and Wizards was considered a taboo; maybe it was also forbidden.

Quoting a dialogue from the movie,

> 
> **JACOB:** Okay, wait. We talked about this, like, a million times. If we get married and they find out, they’re gonna throw you in jail, sweetheart. I can’t have that. They don’t like people like me marrying people like you. I ain’t a wizard. I’m just me.
> 
> 
> -Fantastic Beasts: The Crimes of Grindelwald - The Original Script
> 
> 
> 

For this sole reason, at the beginning of the movie, she left him and throughout it, she seemed to be deeply affected by her choice. Later, when she met Grindelwald in Paris, she surrendered all her defenses. She heard what she wanted to hear. Of a world that she can leave freely without the "limitations" of the past.

Grindelwald, after all, is mentioned to have been an **extremely charismatic** man, who not only persuaded thousands to join him, but persuaded none other than **Albus Dumbledore himself** in his quest to rule Muggles for the *Greater Good*. If Albus Dumbledore was deceived that easy, what chances did poor Queenie had?

The real question would be *why* Grindelwald was so hesitant in making her join him. For that I guess, we will have to wait to find out.