It's not equally as likely that you picked the right door the first time. It's not a 50% chance of one being right.
It is if you just take a snapshot in time, which is what Tarv's doing. Technically, you're right. But once you eliminate one door and then forget
it existed, you're left with 2 choices. At this point, you're first guess has as likely a chance of winning as changing does.
Personally, I like to look at it the other way around. The doors I didn't pick, have a 2/3 chance of being wrong and after opening the third door, it increases the likelihood that the remaining door I didn't pick, is still wrong. In case none of you notice, I like to bend things so they work for me, because I'm never wrong.