Music associated with positive emotions -- love, happiness, and so on -- tends to be written in a major key, and minor keys are used for music meant to sound sad, eerie, or menacing. This is of course very rough notion of a subject that's quite complex and well over my head, and beyond major and minor are things calls musical "modes" that are used in everything from classical music to pop. (This introduction to modes is relatively comprehensible to those of us not well-versed in music theory.)
Anyway, what happens if you take a piece in a major key and shift it into a minor one, or vice versa? Here are two examples, first the happy version of "The Imperial March" from Star Wars (which sounds like it should be the title theme of a classic adventure film), them the creepy stalker version of "All I Want for Christmas is You."
The second version reminds me a greeting card that read something like "I've made up my mind. I want you, I need you, it's got to be you you you!" and on the inside said, "If I can't have you, I want an owl. Or a dirigible."
You can find more examples of major-to-minor adaptations by searching YouTube for "major to minor" (link to search results). There appear to be a lot fewer examples going the other way. That's a shame. I was hoping to find some cheery, upbeat version of funeral dirges. (I'd like the last hymn played at my funeral to be this one.)