Why are people so mean spirited?

I wish I could forgive completely then maybe it would be easiest to forget the pettyness  of others. Not sure why I give so much space in my brain to people who are mean spirited. I think it's my need to fix everyone hoping a smile a giggle or a song can help but some people are so wrapped up in their own anger it just makes them madder to see other people happy. 
I go thru cycles of happy and sad it's very hard because when I'm happy I'm loud giggly and want to sing.a few people at my work place don't like this and tell me shut up. . Be quiet and have gone to management to get me banned from singing or humming at work.do they even know how cruel this is ......