Now that you've succeeded in getting it off, you realize how tight they're on and how much strength/torque it takes to get it off. Since you're already put it back on, I assume you torqued it down pretty tight. It may be a good idea to check and re-tighten the bolt again after it's been ridden a few times. Same thing with the chainring bolts. Re-tighten them again after a few rides.