Assumptions are dangerous things to make, and like all dangerous things to make--bombs for instance, or strawberry shortcake--if you make even the tiniest mistake you can find yourself in terrible trouble. Making assumptions simply means believing things are a certain way with little or no evidence that shows you are correct, and you can see how this can lead to terrible trouble. For instance, one morning you might wake up and make the assumption that your bed was in the same place that it always was, even though you would have no real evidence that this was so. But when you got out of your bed, you might discover that it has floated out to sea, and now you would be in terrible trouble all because of the incorrect assumption that you'd made. You can see that it is better not to make too many assumptions, particularly in the morning.
Daniel Handler