I was excited for Gen 4 as soon as I found out about Diamond and Pearl, as I saw things like the Lucario and Manaphy movie my hype for Gen 4 skyrocketed and it ended up being my favorite Gen. Gen 5 on the other hand caught me off guard, I had thought Gen 4 might've been the end of it because at the time I thought "You can catch God now, where do you go from there?"
I tried to get excited despite the suddenness of it's announcement, but when Black and White finally came and I was disappointed, mainly because I didn't like how much they alienated Gen 5 from the previous Gens. At least Black 2 and White 2 rectified that issue a little bit by allowing you to catch previous Gen Pokemon so I was a little more excited for those games.
Gen 6 I'm feeling very hyped for, maybe not as much as I was for Gen 4, but I do get more and more excited with each new announcement.