The Justice League somehow winds up several years in the future... Or maybe it's an alternate universe? After all, there seems to be an extra kryptonian protector of this time's Metropolis, one that goes by "Superboy".
Alternatively:
Clark is ecstatic to see another kryptonian, even if it's one from a (maybe?) alternate universe, but... his fellow kryptonian doesn't appear to share the same feeling of immediate kinship. Luckily enough, Superman wasn't known for giving up!