Why is it that when something can do wrong, it does?
Confirmation bias. If you expect something to go wrong, you psychologically exclude good events for bad, to confirm your idea.
Why is it that when you drop a slice of bread, it always lands butter side down?
Mechanics. If you knock off a piece of toast from a table, it'll rotate just enough to hit butter-side down when it hits. Solution: eat from a really tall or really short table.
...when you find something it is always in the last place you looked?
Surely that's always true, unless you keep on looking after you've found it!
...when you need something you always discover you lent it to someone else?
Most things you don't need to actively look for. Something you need to look for you haven't used in a while, hence its more likely to be lent out, lost or given away.
....if you're looking for something in a box,it's always at the bottom.
If its something you're looking for in a box, then its something you don't use that often. Hence it's likely to be at the bottom.