I fell in love with him. But I don't just stay with him by default as if there's no one else available to me. I stay with him because I choose to, every day that I wake up, every day that we fight or lie to each other or disappoint each other. I choose him over and over again, and he chooses me.

This quote means that the person fell in love with someone and their decision to stay with them wasn’t due to lack of other options, but because they consciously chose to stay with that person every day, despite any fighting, lying, or disappointment. Every day, they make an effort to choose that person, and that person chooses them back.