I'm sure this has happened to you as well: you are reading some hand-written work, the variables used are $u$ and $v$, and at some point the handwriting becomes unclear and you cannot distinguish the $u$s and $v$s at all. This happens to me quite often --- even distinguishing them when they are typeset can sometimes be irritating.
Is there any particular reason, perhaps historical or traditional, why the two most similar-looking letters in the Western alphabet are also one of the most frequently used pairs of variables?