Well technically we came into existence because before we were born we didn't exist, perhaps we aren't even 'living' now we are just a bunch of say data flowing through a computer, the data being our consciousness and the computer being our bodies, data can forever be lost(we die), duplicated and placed in a new computer(cloning) or moved to a new computer(placing our consciousness in another body, though that technology hasn't yet been invented).
I'm agnostic in every sense, don't know, don't care, but there is a possibility reincarnation doesn't exist(every lifeforms on Earth, nay, in all existence can be killed, so we wouldn't be able to reincarnate, making that theory obsolete). As much as believing in afterlife would be nice, I always try to take the rational thought, and consider even thinking about it except to share my thoughts with others such a waste of time, I never even think about it unless someone else mentions it, and guess what, 5 seconds after I enter 'post' I'll forget about it again.