I don't really subscribe to the "Mystra groomed Gale" fan theory (we know Larian is playing fast and loose with official Faerun timelines, but she only reappeared in 1487). Their relationship was more along the lines of a PhD candidate and their department chair is one of the top researchers in the field and their advisor, who initiates the relationship. (Or maybe he's among the top researchers, but she's the one who controls funding and grants and publishing and conferences.) He walked into that relationship flattered and thrilled and proud, only to realize after a while that Mystra is holding his metaphorical grades and thesis and future research opportunities hostage. And what is he if not his genius intellect?

Mystra's attention was flattering, but she could just as easily move on to the next hot young genius with the Weave. So he has to do the only thing he knows he is good for: be a genius with it. He wants knowledge (don't we all), she's rightfully gatekeeping it, and he knows that if he pushes, she'll toss him aside, frustrated with him. We get the impression that she only told him to stop wanting, but didn't give any reason why. Maybe Gale, in hubris, didn't listen. But no matter. Gale is convinced he's only good for his Magic, and Mystra is only with him for it, so he tries to do what any of us would do: impress her.

Why does Gale believe that he's only good for his Magic? Because society groomed him. Society viewed him as the genius wizard. Teachers only praised his abilities. He gets attention not for his other skills or personality traits, but because he's an eight year old who can cast Fireball with ease at at time when Mystra has barely started reconstituting herself. Society taught him the only thing people noticed was his skill. Look around at how many former gifted kids hit walls doing undergrad or grad school, don't understand how to take notes or how to study because it's always been innate, and burn out. Being able to read collegiate materials is no longer impressive when everyone is reading collegiate materials. A huge part of the identity of gifted kids, praised since childhood on their academic talents, gets lost when they hit this wall or realize they're no longer special. (Trust me, being teachers' pet and doing top-notch academically gives you an ego. Doubly so if your family are academics, which we have some evidence that Gale's family is at least moderately wealthy and/or connected with other mages.)

Is Mystra's relationship with ANY of her Chosen a good thing? I'd still say no, unless they both can keep it platonic and mentoring. (I had a great professor in college who did just that: mentored me. My high school physics teacher is still a friend. He was a guest at my wedding.) As soon as romance or sex enters that relationship, it's gonna be a bad time, even if the romance is one-sided on the part of the mortal. It's inherently difficult, shaky ground for a relationship, because of the power dynamics. If you piss her off enough, she can take your connection to the Weave away, and the closer you are to the situation, the more likely you are to think someone deserves punishment. And if she does punish you, it feels like not just a fall from grace but also like you are particularly a problem. There are evil mages all over, and Mystra hasn't taken their powers (the Weave is True Neutral, but still...you think every mortal mage is going to understand that if you eliminate the evil users it actually destabilizes the Weave?). She's the only god who can't really punish those that dramatically deviate from her alignment, because of what she is the god of. Think about that. Every other god can say "you are not following my rules, get out". She can only say that to her clerics and paladins (5e can kiss my ass, paladins need a patron to hear their promise), maybe to her Chosen, but certainly not to anyone who uses her power.

I love Gale, but this really puts things in a different light.