It seems clear now why Daniel was crying in the future. He is seen crying as the rescue crew uncovers a fake version of Oceanic 815 and doesn't know why. I think that Daniel has "mind" time travelled so much through experimentation that his thoughts and memories are all mixed together causing his "condition". He seemed pretty convinced that he could now change the past and cause the 815ers to not crash on the island. But now in the future he sees that their plane crashes in the ocean.
Perhaps he is crying because he thinks his plan got them all killed. Instead of the 815ers being alive on the island he thinks they are now dead at the bottom of the ocean. All thanks to his plan. Hence the sadness.
Perhaps he is crying because he thinks his plan got them all killed. Instead of the 815ers being alive on the island he thinks they are now dead at the bottom of the ocean. All thanks to his plan. Hence the sadness.
