I have a sinking sadness about happy things ?

For quite some time now, (maybe a couple years) things that would considered cute and heartwarming I get sad about. Like one example I cried during the wizard of oz when dorothy, the tin man, the lion and the scarecrow were leading down the yellow brick road towards the emerald city, and little toto following them happy wagging his tail. I cried. I felt depressed about the dog. I dont know y. There r so many other examples. Like when I think of my dad as a little boy I get sad. I am a 14 yearold girl. I dont know what's wrong. I might need help but all I want is to not feel saddened about cute things. Winnie the pooh gets me depressed too. Help me!