ask PI for more acknowledgement of good work / affirmation
This sounds like a matter you may need to take into your own hands. You can't make a person more emotive, affectionate, cheerleader-eque...that may just not be your prof's style. Also, you're sort of asking them to do more "work", even if you don't see it as work. It's one more thing they have to deal with. Why not solve this yourself: be the change you want. 

Do you have a pin-board in/near the lab? Cover it in bright paper, call it the achievement board, and start tacking announcements of jobs well done. Alternatively, create an news/achievements email that goes out to everyone once a month that highlights what everyone has accomplished and make sure everyone tells you what they're up to. 

Just two of many possibilities, but this way you come across as proactive to your prof, a champion of your peers, rather than needy and demanding of more of their time.