Nihilism is only pessimistic because we as a society want everything to have a meaning.
People feel like having a religious belief naturally. It's kind of in our genetics... well for some of us at least.
It's even partially backed up by science.
You know when you see barmy ghost hunters with electrical/magnetism sensing equipment... well it works but not for the reason they think.
It's been tested and it kind of works, if you use magnets to make a magnetic field in a certain area of the brain it stimulates electrical activity in the temporal lobe. People can get a variety of result from full blown relgious visions, to a feeling a presence that is on another plane of existence.
The researcher went to one house where a young girl was seeing ghost's, found in the house a magnetic disturbance was caused by a bedside radio clock, removed it from the room and the girl stopped having the visions.
In fact those with Temporal Lobe Epilepsy (aka naturally increased electronic activity in that area of the brain) do tend to get religious visions, even those who are atheist, one atheist who had temporal love epilepsy saw himself in hell for not believing in Christianity (and he stayed a atheist afterwards). Some even theorise many prophets in religious documents actually had temporal lobe epilepsy.
Some fairly recent prophets of fairly recent american churches have had signs of Temporal Lobe Epilepsy
On the other side of things those with naturally less sensitive temporal lobes tend to be less religious. For them even if you manipulate the magnetic field around the temporal lobe they will not feel or see anything different.
It actually kind of makes evolutionary sense seeing throughout history religious people have tended to live longer.
Anyway!!!! Point being, many people like to believe in something naturally. The suggestion that everything is for nothing is pessimistic for anyone, especially those who have invested time and belief an a "higher cause".
I have been a nihilist for a very long time, but only recently realised it (I only just found out about it's real "name"). Even if there are higher forms of life it would most likely a similar comparison to Humans and Bacteria... with us being the bacteria.
I'm also a moral/ethical nihilist, if you look at every negative or positive action in our society, there will be another at some place or time that would have a vastly different view on it. As soon as you have one person disagree with a moral/ethic it no longer becomes a moral/ethical standard.