Even the end times stuff in the Bible is only pessimistic if you subscribe to premillenialist views. This might be veering off the subject, but Catholic/Orthodox and older Protestant views typically have been preterist and less literal in their readings. Which is to say, they believe that the "end times" scheme in Revelation was highly symbolic, and that the millennium spoken about where Christ rules on earth for a "thousand years" is actually the present - the age of the church. When it's put in terms where "Christ rules on earth" people automatically assume it's meant to indicate some period where there's absolute heaven on earth, some state of perfection to look forward to, after God wipes everything out. It never said that though. And the church didn't interpret it that way either. It recognized life as something to actually be involved in - yet didn't want to get too attached either (Augustine's City of God is a lot about this subject).
In other passages, before he died, Christ tells his followers that they'll do greater things than even he did. Another indication that he was here to try to build something, not destroy, and saw a lot of work to be done even after he was gone. There's also this famous passage where he puts the burden of change on it's hearers, and downplays future expectations (or pessimism) - “The kingdom of God does not come with your careful observation, nor will people say, ‘Here it is,’ or ‘There it is,’ because the kingdom of God is within you.â€
So in one sense, Christianity once had a stronger, optimistic point of view, and saw itself as a positive presence in the world. It did not hope that it all went away, and the majority of humans fried in hell, like some modern preachers do. Although some modern Christians still have the optimistic spirit, of course. What were Martin Luther King's most famous words? "I have a dream." He didn't say "I have a nightmare." Unfortunately, some people politicize his optimism and call it merely "liberal". Which he was.. But I think people like him carry some of the actual genuine elements of Christianity myself.
Anyways, not to be Christian specific or anything. I think a lot of religions are optimistic.