Recently I missed an opportunity in a purchase that most likely won't come back up again anytime soon and after beating myself up over for a bit it all came back to how being ready when the time arrives is so important to many things, especially fishing.
I learned this a long time ago as a guide and really clarified it one day when fishing with Capt. John Kumiski for his book Redfish on the Fly a Comprehensive Guide. The other guy in the boat that day said something about luck and fishing, spurring John to tell a story which is longer than I'll type here but the point was that effective fishing has nothing to do with luck and everything to do with being ready for whatever you encounter. The first example that comes to mind on this is having a fish show up just 50 feet out only to realize you aren't ready because the fly line is wrapped around your left leg or fish are chasing bait up in exposed oyster shells but you don't have any flies weedless enough to get close in.
I don't think this means that you have to take the kitchen sink with you but it does mean that you need to understand your target and be able to take advantage of the situation. The better you are at determining the possible demands the better you will be in selecting what is required to meet those demands.