Rob's absence at the reunion -- and brief returns to the show in the final season -- was addressed by Khloe, who shared that her younger brother "just needed a break" from the reality TV lifestyle.
"It's not so much about physical appearance," she explained, "it's about how he felt internally about some of the women in his life, and how they treated him or how he felt they might have used him to get to certain areas or levels."
"I think that really affected him, because my brother is such a lover, and has the most incredible heart and personality," Khloe continued. "I think he's just getting stronger. I think you have to get heartbroken and learn from your mistakes, and I think he's just stronger and feeling better to be around, and be more vulnerable or exposed."
Rob stepped back from the spotlight several years ago, following his public relationship and split from Blac Chyna -- with whom he shares a 4-year-old daughter, Dream -- and a health battle with diabetes. A source told ET in March that the youngest Kardashian sibling is now "in a really great place."
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Rob Kardashian Is Dating Again But Daughter Dream Is His Priority, Source Says
"After his daughter was born, he committed to taking better care of his health so he could have the energy to keep up with her, and the benefits of that are visible," the source said at the time. "He looks good and seems happy. This wasn't about a quick fix."
During the reunion, Khloe echoed that her brother is "feeling really good about himself right now, and he's working really hard on himself," but noted that Rob isn't quite ready for a public event like the reunion, where he'd have to "address the parts that made him want to take a break."
For now, the family said that Rob is dating again, and working on co-parenting with Blac Chyna -- who is currently embroiled in a legal battle with his famous family over the cancellation of their E! spinoff series, Rob & Chyna.
"We can only imagine how hard that is for him, and I know he feels really guilty about that, so none of us ever make him feel bad about it," Khloe said.
"Dream is honestly one of the most incredible little girls in the entire world," she added of her niece. "We never blur the two... We respect that Chyna is the mother of Dream, and we'll always respect that that's her position. So we never try to [let that bleed through] and make Rob feel more guilty. 'Cause that's out of his control."