Why NOT end a sentence with a prepasition?

ARGH - what am I missing? He didn’t want to marry the warden’s daughter because…? Jeez, I was so enjoying the thread, and then this. He didn’t like the plan because, then he’d be the warden’s son-in-law? …because…SHIT! I can’t figure it out. He didn’t want to preposition the warden’s daughter? He didn’t want to end his sentence with a proposition? Hey - maybe that’s it? I can use some help here…