Why do people cheat on their weight loss journey?

Pepole say no more junk food or whatever but they eat it anyways. People never follow the rules at all. My mom said, "So what? We gotta have a cheat day sometime. Nothing wrong with that."
I was like, "Mom we're suppose to follow the rules in weight loss. Not to eat junk."
My sister is like, "Oh come on Lila (me), you don't have to follow EVERY rule. It's ok to bend the rules once in a while. Quit being so uptight! Losen up!"
I wish people would ever so stop saying I need to losen up. Why don't people ever follow rules.