Well, that depends. Intuition serves people in many ways, but the question is: is life in general a negative thing or a positive thing? (I'm operating under the assumption that truth is constant and certain regardless of how we perceive things). In the case that our existence is truthfully overall a positive thing and is significant and has meaning, than I believe both Ne and Ni are going to perceive it that way. Si and Se may not because they both look at concreteness or the here-and-now, but I think iNtuition looks deeper and/or more broadly at things to exhaust possibilities and totally explore every facet of life, so inevitably they would reach whatever the most true conclusion is. If our existence is in truth a negative thing that has no meaning (if the nihilists are right, in other words) than I believe Ne and Ni would perceive it to be so. So whether one function is pessimistic or optimistic depends on the truth of the situation.