"It doesn't happen to you, you happen to life" means "rather than waiting for events around you to dictate your actions (i.e. life happens to you), you boldly act and make an impact on the word (i.e. you happen to life)".
It's not phrasing you're likely to see except in poetic phrasing and extremely casual speaking/writing.
It means to take advantage of every single wonderful opportunity and challenges that come your way and make the best out of your life. You are who makes the difference because because we're all unique and made tof shine as ***'s creations. I don't know where you heard this quote or linext but I'm familiar with it from a song by Superchick: