Great example. Let me build on that: you meet someone who initially slots right into that schedule at first but then their situation changes and they are only available on weekends. You've already built a significant relationship with them that you want to maintain and that means you have to share your weekends.
But you know, it's other stuff too. Things that might not seem a big deal now or you can't imagine wanting to do or feel for anyone else crop up. When you make those kind of promises to someone to protect your relationship and then want to change them later, your partner can mistakenly assume your feelings for them have changed.