Holy crap I juz realized why tf the old Jack is so serious

Cause the old Judy was supposed to be a serious and focused on work character when Nick was the protagonist for the original Zootopia. 

And before the old Judy was Jack. 

aka Jack was supposed to be serious and just focused on work

HOLY CRAP I FIGURED IT OUT 

But it still doesn't explain why Jack's such an ego egghead and a player