"It doesn't hurt to ask," is not an act of bravery. More often it is the crutch of the inconsiderate. Sometimes it does hurt to ask. Edited by @ebsquared
  1. They have already agreed.
    If the terms have been negotiated, going back to ask for more is disingenuous and trust-breaking. And it weakens your negotiating position. The other party is aware you might ask for more and they compensate by only agreeing to terms that provide an extra buffer.
  2. You are making them say no.
    If you know will probably say no and ask anyway, you are being inconsiderate. No one likes to say no. This is a form of peer pressure and results in people saying yes when they shouldn't.
  3. You are just being cheap.
    Trying to avoid a nominal fee? By asking, you're making someone pay for something you want. If you want it, pay for it.
  4. It's going to cost them.
    Indiscriminate asking costs someone finite resources. It may be time, money, or energy, but they won't be getting it back.