Question

why should the percent yield never be above 100%

Answer 1

Well, your theoretical yield is 100% and that's under ideal conditions which can never be truly met so your percent yield should be somewhere close to 100% but not exceeding it.

Some possibilities as to why your percent yield exceeds 100%:
1. You accidentally put more of the reagents in than you thought.
2. You've done your calculations wrong.
3. Your "product" is impure:
3a) You weighed it while it was "wet" and therefore included the weight of some of the solvent.
3b) Whatever method you used for purifying the product didn't go so well; if you're crystallizing the product, you've still included impurities and you should recrystallize.

Answer 2

law of conservation of mass