I love listening to other people's conversations it can be very amusing. Obviously like you said, I don't purposely do it, but times like when you're on the bus it can be so funny.
It doesn't seem weird at all. You can't help if you hear what loud people are saying. I remember one time my sister and me were walking out of Target and out of nowhere we both hear: "HAHA I GET IT!" It was so funny. These two girls were walking behind us and one of them just said that out loud and when we got to the car we burst out laughing and said " HAHA YOU DON'T GET IT!" It's still funny today And almost everytime I leave Target I expect something like that to happen again.