system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] Why doesn't Darth Vader stop Luke from falling on Bespin?

When Luke let's himself fall after finding out Vader is his father, there is nothing stopping Vader from just lifting his body and putting him back on the railing. It certainly isn't a hard feat for him. So why didn't he?
gpt
It's important to recognize that Vader had a significant change in plans during the fight against Luke.

Initially, his plan was simply to capture Luke, bring him to Palpatine, and let the Emperor turn him to the Dark Side. He attempts this by dropping Luke in the carbonite freezing chamber and activating it, but Luke leaps out and quickly proves that Vader has underestimated him.

At this point, Vader realized that Luke was very powerful despite presumably having no training. (Vader was unaware of Obi-Wan's Force Ghost and the fact that Luke had found Yoda). Perhaps he could become powerful enough to challenge Sidious. This made him consider a new option: have Luke help him overthrow the Emperor. Vader suggests this to Luke, dropping the "I am your father" bombshell in the process.

So **Vader actually no longer intended to capture Luke at this point, but instead to recruit him**. Taking Luke by force would no longer serve his purposes. Instead, he gave Luke a good reason to join him, then allowed Luke to escape and take some time to consider Vader's proposal, as well as come to terms with the fact that Obi-Wan had lied to him. Vader hoped that Luke would, after some thought, abandon the dishonest Jedi and accept Vader's offer.

If Vader had simply taken Luke against the latter's will at that point, rather than treat Luke as an equal capable of making his own decisions, then Luke would have been much less likely to work with Vader against the Emperor. And if he had simply continued with the previous plan of freezing Luke and bringing him to Palpatine, then there would be a very good chance that Luke would reveal Vader's betrayal upon Sidious' subsequent interrogation, once he was unfrozen.

Vader's decision was actually the best option under the circumstances. Unfortunately, he underestimated Luke's faith in Obi-Wan.