To express obligation we have two verbs in English: have to & must

- What's the situation right now, on this road trip. What... we were just...We were just at McDonald's, what did you just say?
- I have to go to the bathroom.
- You have to go to the bathroom?
- Yeah
- We were just at McDonald's like 5 minutes ago
- Yeah
- Why don't you stay there when we were there?
- ++++
- What? Take that chicken ring out of your mouth.
- 'Cause I didn't have to* then.
- But you have to go five minutes later.
- Yeah
- Ok. That's the story of my life.
- Bye.

to = to go to the bathroom
'cause I didn't have to then = because I didn't have to go to the bathroom at that moment.