According to music theory, songs in minor keys are supposed to be sad. However i often find minor pentatonic songs, especially from Chinese folk songs, that are happy and upbeat (Chinese New Year songs like Gong xi gong xi comes to mind). Any reasons for this "deviation"? Thanks in anticipation ☺
It is more like a preferred usage and style than being a rule. You can find cheerful songs in minor and dramatic songs in major especially in Baroque and some classical music derived from folk music. Lots of style elements and personal preferencese like harmonic progression, melody structure, use of intervals (especially minor 2nds and pentatonic modes doesn't include them), instrumentation, mimicking...etc are very effective in this effect.
For example in Turkish classical music (the authentic one) there are many full 7 note scale modes (Makam) with unequal comma intervals. Turkish makams also come with style rules, so there are different Makams with different feelings using exactly the same scale. This is because different usage brings different interpretations, feelings, atmosphere...etc. Transposition of a Makam is also a different Makam because with the register the expression and stylistic usage is also altered accordingly.
Same goes with the minor and major modes if you introduce different style rules than the accustomed ones.
I would suggest Irving Berlin's "Blue Skies" as an example of a cheerful song in a minor key. However, it can also be viewed as an extended musical pun on the term "blue" as used in the linguistic phrase that is the title, an indicator of happy circumstances, contrasted with the term as a style of music. A masterful piece, it served as inspiration for Monk's "In Walked Bud." Both tunes are worth a listen to give you examples from the American Song Book on one hand, and the Jazz repertoire on the other.