When human existence is meaningless, why human going to find a meaning for it? Every meaning will be pessimistic... because human will die someday, a state will fall someday, the end of the world will come anyday, lover may die in the next day... everything is so unpredictable. To become optimistic, human must know everything will decay eventually, and human existence is only for the sake to be exist, then what human can do is be happy, and do not do anything regretful.