I am struggling to find the connection between pitch and music when hearing others sing. Any advice?

I am watching singing videos on youtube from audition tapes to celebrity performances, but I am struggling to find the connection between the singer's voice to the background music. Now I understand in singing, you are supposed to match the frequency of the music, but I just can't hear it. I also tend to hear singers who other's claim are off pitch, but I think they sound pretty good. I've been practicing singing for a while, but this is really holding me back.