Except that \left\right is almost always the wrong solution. It's one of my pet peeves when collaborating on papers. Your parentheses shouldn't scale all the way up and down your equation, unless you want them to be disgustingly tall (just try \left( \int_0^1 \frac{dx}{x} \right) to see what I mean). You should choose \big[lr], \bigg[lr], \Big[lr] or \Bigg[lr] based on what looks good, and the computer is not good at guessing which one will be visually pleasant.
in latex, when typesetting math, by default, parentheses (and other brackets) are always a constant height. So if you put something which is taller than one line in between parentheses, it will look strange, with the content sticking out past the parentheses at the top and/or bottom.
The fix for this (in latex) it to mark pairs of matching parentheses with the macros `\left` and `\right`, (for example: `\left( \frac{x}{y} \right)`) which informs latex that it should figure out how tall the content between the brackets is, and then resize the parentheses appropriately.