Ben learning that Gwen and Kevin are leaving that of Bellwood, leaving him a loner.
The reason behind Feedback's lack of usage by modern day Ben....as during the course of Ben's Summer, Feedback was Ben's most favorite alien...only it starts to be seen as Ben starts craving the energy rush rather than choosing any other alien, which Azmuth, Max and Gwen call him out for it, much to Ben's anger...which proves to be the Fatal Flaw for Malware to RIP Feedback out of Ben, destroying the conductoid into dust, Ben, enraged and stricken to tears, lets out his anger to give Malware a full on overload of the omnitrix's energy, causing the corrupted galvanic mechamorph to explode. Ben won...but at the cost of loosing one of his favorite aliens...and the rest is history
This event was so traumatic it stopped him from telling it to Rook, who Ben considers a good partner and best friend by Omniverse.
Ben 23 acts like a rich show-off, but is actually a way to fill the void with fortune and fame after the death of grandpa Max before he got the Omnitrix.