"Pride goeth before a fall" is a famous Biblical proverb. Westeros doesn't have the King James to consult, but it's probably a good bet The Seven-Pointed Star has something vaguely like it. Unfortunately, Daenerys never really was brought up on Westerosi religions, despite her brother's hair-brained insistence the family would one day take back King's Landing. She already put her dragons in danger once with her hubris and lost Viserion to the Night King. Now she's made the same mistake twice when Rhaegal is killed on Game Of Thrones by Euron. Warning: Spoilers for Game of Thrones Season 8 follow.
The fact that both Rhaegal and Drogon survived the battle with the Night King was a miracle in and of itself. Moreover, the dragons are a crucial part of Daenerys' plan to take back King's Landing. They're the weapon she has no answer for.
Except she does. As viewers will remember, Qyburn did some research and rebuilt a weapon known as "the Scorpion" a version of which was used back during the Targaryen Civil War, known as "The Dance of Dragons." Such a weapon didn't exist 300 years ago when Aegon first arrived on Westeros.
Bronn attempted to use it on Drogon at the Field of Fire, but his aim was off, thankfully. Therefore, Daenerys knew it was out there, and she should watch out for it.
And yet, she did not. Instead, she convinced Jon not to ride Rhaegal. This was because she didn't want him seen by those in the south as somehow her equal. (A dragon with a rider is always in less danger than one without) It was a foolish choice if ever there was one.
Euron's aim was far better than Bronn's ever was. In fact, he practically bent those arrows like Beckham as he hit Rhaegal in the chest not once, but twice, causing the dragon to crash into the ocean and drown. And with that crash went another piece of Daenerys' heart.
One would have thought she would have learned from the trip North to face the Night King. One would have thought she'd be more careful coming south with her army. But victory begets hubris. After all, Daenerys had led her army to a win over the Night King, a far deadlier foe than Euron and Cersei put together. What did she have to fear from them?
But dragons, though they are magic, don't make anyone bulletproof. The dragons didn't win the fight against the Night King. Arya did. Dragons are a piece of magic that helps those who help themselves. They are a tool, like any other in Daenerys' arsenal. And now, since crossing the Narrow Sea to conquer Westeros, she's wasted two of them.
Worse, the loss of Viserion and Rhaegal is, to Daenerys, like Cersei's loss of Joffrey and Myrcella. Each child who dies makes her a little harder, a little more bitter, and a little more likely to go completely mad. King's Landing should pray they don't pay the price for Rhaegal's fall.