One could argue differently. One could point out that at the beginning, Buttercup was a complete, bullying arsehole to Westley until she realised he loved her (making him something of a masochist).

Having finally decided they're in love, he goes off to seek his fortune (because it's that kind of story) and she promises to wait for him. But he gets taken prisoner. By the time he realises that he is not being imprisoned but trained, and duly takes on the identity of the Dread Pirate Roberts (another familiar trope). Buttercup has broken her promise and is engaged to Humperdinck, She didn't run away, knife him or kill herself, he just accepted his proposal. For all Westley knows, she's forgotten all about him, or never really cared. Nevertheless he overcomes a series of challenges to rescue her from Vizzini. Climbing cliffs, duelling with a master swordsman, fighting a giant, and drinking poison.

No wonder, having done all that, he's more than a little pissed off and equally anxious to know if Buttercup still actually loves Westley. Apparently, she's still enough in love with Westley to attack the Dread Pirate Roberts when he taunts her about it. Because it's that kind of story.

It's as daft as a brush and wholly unrealistic, but it's light as a feather and nobody takes it seriously.

Except you, apparently. But it's not compulsory to like it, or watch it. Nor is it compulsory to sermonise upon it like a Puritan preacher inveighing against public laughter.

With all due respect, there are far more important things against which you could direct your ire and pen. I look forward to your doing so.