There is nothing more depressing than having it all and still feeling sad.
what does refer "having it all" in this quote?
It means fortunate or something else??
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This is a manner of speaking in some places. I don't know if it is the same in every English speaking location, but it is so in US, Canada, and UK.
It refers to the idea of having everything you reasonably desire in a context.
One possible context is a man having a good job, savings and investments, respect in his community, a great family, a nice home, a nice car, still healthy, and at a relatively young age to have achieved all that. The "all" here is all the things and conditions that might be considered desirable.
There are many other possible contexts. The idea is that to "have it all" is relatively uncommon. It is often found that one or more of these things is lacking. For example, the man might have worked ridiculously hard to get all of the other things, and damaged his health. Or he might have needed to work until he was old to get them all. Or he might have all of the visible things but be deeply in debt instead of having investments. Then he would not have "it all."
The idea of the quote is that a person has all of those desirable things. He ought to be happy about it, since he has all of the outward signs of fortune. Some reason for sadness has arisen that is more powerful than all of those things.