In Barcelona's Champions League match versus PSG, Lionel Messi started out on the bench, not on the pitch, something that he's very much unaccustomed to and it showed. Watch how he fidgets and gets uncomfortable, especially after PSG's Pastore scores a goal. I'm sure Messi would go crazy if he wasn't inserted into the match when his team is losing.
But Lionel Messi eventually made his way to the pitch at the 62 minute mark and helped create Pedro's goal just ten minutes later despite his thigh/hamstring injury. Messi just loves to play soccer and it tortures him not to be out their playing. I'm sure it doesn't matter if it's a pick-up soccer game on the streets or a Champions League match at the Camp Nou, Messi just wants to play.