Assuming it's not due to trauma like an accident, stroke or use of forceps during birth, you can find out more about it by looking up Emotional Lateralization. Basically though, it's just* a quirk. Looks like neurologists are still at the, "we're still collecting data" stage, meaning they have theories but nothing they can prove definitively.
*meaning it's not indicative of something you need to go to the doctor to have checked out immediately. Like KK said, though, if it concerns you it wouldn't hurt to have a doctor give you their opinion.
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“For small creatures such as we the vastness is bearable only through love.”
―Carl Sagan