They can be happy tears or sad tears.
For me, it's the last page and a half of Memnoch. Although I'm not the biggest Lestat fan, his agony at his station in life is depressing. It's also interesting and sad to see him, for once, become pensive and disturbed enough to need to back out of the spotlight, to see him revert to a quieter state. I never truly appreciated Lestat until I considered him not being around anymore. The line "Let me pass now from fiction into legend," breaks me up every time.
Also, Louis' attempted suicide. I remember reading it for the first time on a car trip to visit family, and I started sobbing so ridiculously that my mom thought something was seriously wrong. It still is heartbreaking to read. Louis' another character I didn't much like until I imagined him gone. It was that book, also, that made me fall in love with his beauty. When he and David are hunting congenially together, you see Louis in a new light that makes you smile. At least I did.
As for happy tears, it would be in Queen of the Damned when Marius and Armand meet up again after all that time. It's so touching. -Sighs happily.-