6. You're not prepared for his answer
His answer will be either yes, no, or maybe. Don't forget this will be a two-sided conversation and you don't know how he's going to react. You should know what you're going to say to any of these 3 choices:
If he says "yes", it's time to stop talking and move on to other things. If you don't, you might end up talking him into changing his mind.
If he says "no", you need the reasons why he should say yes at your fingertips. Be prepared with strong, logical arguments to help change his mind. Remember, this is important so you are not going to just accept a no without a "fight."
If he says "maybe", you have to give him a good reason for what you're asking for and then stop talking. Change the subject and give the man time to think things over.
Decisions are hard to make on the spot and giving him time to ponder your request can do wonders.