It's easy to get so caught up in following pattern instructions that you lose sight of what's really important: gauge. You've probably already purchased the yarn for your project (or decided which of your stash yarns you'd like to use) by the time you sit down to work your gauge swatch, and you've probably just grabbed the needles or hook suggested by the pattern.
That's a great place to start, BUT the important thing to remember is that the needle/hook size given in a pattern is just that: a suggestion. All it means is that the designer (or possibly the pattern tester) got the gauge specified in the pattern using that particular combination of yarn and needle. Don't forget that everyone knits or crochets a little differently, and you may need to go up or down a hook size or two -- or even more! -- to get the correct gauge.
For more on how to make a gauge swatch (and why), please see this FAQ.