Why is it that one side of my lip(s) goes up more than the other when I smile? (Photo)

i like to think i smile just fine, but sometimes it really bothers me when i take into small detail why my lips do this when i smile. my left side has more of an upward curve to it when i smile and the left side, though its not completely flat and weird looking, it isn't as lifted as the other side. I'm wondering is this a genetics thing or some possible thing in the past that i don't remember or am not aware of could've caused this?