Under Ego's guidance, Star-Lord began familiarizing himself with the power inside his body. Perhaps because that power came from Ego, the more familiar Star-Lord became with it, the closer he seemed to grow to Ego.
Especially after Ego and Star-Lord, both far too old for it, started playing a little father-son game of catch, their relationship grew better and better. Star-Lord felt that Ego probably really did want to be a good father...
That was what Star-Lord thought at the moment, but Gamora, on the other side, did not think so. She kept feeling that Ego did not seem like some loving father. At the very least, Gamora could sense that Ego's attitude toward Mantis was not exactly good, much like Thanos's attitude toward Nebula.
Of course, it could not be denied that Ego might treat Star-Lord the way Thanos treated her, giving him more genuine feeling. But even blood ties needed time spent together before emotions could deepen. Ego had been so excited the moment he first saw Star-Lord, and that was clearly somewhat abnormal.
Moreover, when Ego had told his story earlier, his claim that Star-Lord's mother had made him fall in love at first sight had instinctively made Gamora somewhat skeptical. So Gamora wanted to investigate the situation on Ego Planet, rather than hear it all from Ego's own mouth.
So Gamora suggested leaving Ego and Star-Lord space to talk, but she had not expected Ego to react so quickly, directly having Mantis follow them. With an inside watcher like Mantis around, Gamora did not think she would be able to discover any key secrets.
"You're not a pure human, which means your mother was a bug?" But before Gamora could even think about how to send Mantis away, Drax looked at Mantis and started asking her,