I think it's clear from Sterling's own comments that he wanted to play more, and wasn't happy when he didn't play.Personally, I hate these aggregator quotes when the full context of what Pep was saying is lost. Simply, it was Pep talking in generic terms yet again about "happy face". Something he has done repeatedly over the years.
Wonder why you've posted it on here? Do you think it was specifically about Sterling?
As many have pointed out, as far as our attackers go, he's actually been one of the most used in the last couple of seasons. However, he's someone who rarely gets injured, and is known to recover quickly between matches, and had previously been playing a LOT more than most of the squad.
I suspect for Sterling that he's fit and raring to go every game, he's had a couple of seasons where he was the main man, and was on the verge of being considered one of the top few players in the world. It's no doubt frustrating that he played less in the next two seasons, (whether due to a slight drop off in form, or Foden, Bernardo, Mahrez improving around him, or the change in tactics), and clearly he feels that's mostly not his fault.
Given he was the longest serving member of the squad it's not a surprise that he's a little frustrated and wanted to move on for a fresh start.