ELI5: How do income tax returns work?

Why do I have to pay $10 to the government? I'm 27, full time employment making 50K/year, with 20K in outstanding student loans (but I pay at least my minimum every month). I don't own a house, I rent in downtown Toronto. It's the first time in my life I've actually owed the government money and I don't understand what I did differently in 2015.