Why can 'no' not sometimes be 'yes'?

I'm not a big James Dobson fan, but one thing he said years ago has really stuck with me.

As parents, sometimes we just get in the habit of saying 'no' to our kids.

Can I play with your iPhone? No
Can I have some ice cream (30 minutes before dinner)? No
Can we stop by the gas station and get a Coke? No

Many times 'no' is the right answer, but does it always have to be?

Sometime soon, when you would normally say 'no' to your kids, try saying 'yes'. It'll be fun to see their reaction.

Is there really a good reason why 'no' cannot sometimes be 'yes'?