Taking the question literally - what song "has changed the way that you look at things" - the first song that comes to mind is "Band Of Gold" by Freda Payne. I used to listen to that song on the record player all the time. I don't know which of my brothers or sisters had actually purchased the 45, but I adored that song. For me, it was the song that made me receptive to the great soul music of the '70s, to the disco songs of the late '70s, and to the elemental power of a great melody. But more than all of that, it made me realize at an impossibly young age that music could be ecstatic and brilliant and make you want to dance while at the same time it could convey the deepest of pain.
I have never lost my respect for music. I have to laugh when people dismiss some songs or singers as cheesy or amateurs and embrace others as modern classics or true artists, or when people argue about who is a better singer or what is a better song, because they miss the point entirely. Music can't be quantified. A Celine Dion single that might make one person turn off the radio in disgust might make another person burst into tears or remind someone of joyous memories. Someone's Lil Wayne track can be as powerful to them as someone else's Tim McGraw single. It's all wonderfully subjective.
And that's because music is so complex and amazing. And for me, personally, "Band Of Gold" was the first song that showed me how a song can break your heart and heal it at the same time.
I love your question because the answer is infinite, just like music is.