Sadhguru said "Pessimism or Optimism, both are attitudes; either way you are denying reality."
Let me try to interpret what Sadhguru says.
The reality referred by him here means the ultimate truth of divine - the God which is manifesting in all names and forms which is invisible to deluded people who can't recognize Him.
There are people who believe that there is no God who are called Atheists and those who believe are Theists. One may call theists as optimistic and atheist are pessimistic.
These are two different 'attitudes' which is a mind game. Sadhguru in a subtle way says, " Unless one goes beyond the mind and experience the divine through meditation you will waste time in arguing who is right."
One cannot experience God like tasting a banana as He is far beyond senses. your 5 senses cannot show Him as God is not a sense object. Hence mind & intellect has no power to comprehend Him as they are meant for using words to defeat the believer. The only way one can recognize God is by transcending mind & intellect through meditation. In essence "When ego disappears, there you find the all illuminating divine".
In summary don't waste your time in analysing God; but get onto deep inside your self to find Him.