Yes, probably how you feel about it depends a lot on how you feel about the overall ending. I'll have to spend some more time pondering why tragic endings can appeal to me. For example, I also really liked the ending of BBC's Merlin, which was arguably even sadder than Guardian. I'd also throw the LOTR in there, with Frodo's "death." I think part of it is that they pack such an emotional punch that the feelings linger with me and make me think more. Also, perhaps in some ways they feel more realistic to me as opposed to a HEA ending in that they capture the bittersweet/sad aspects of life. Certainly with both Guardian and Merlin, I like the tragic ending because it's not just a "ok, I don't know how to end this, let's just kill everyone off" and not just done to be depressing but rather the characters' sacrifices are for a real purpose.

But anyway, as you say, everyone has their own feelings on the subject!