You'll find that nearly without exception, question form in modals is little appropriate.
On content vs interaction
Probably to core duality in all interfaces is the that of content and interaction. Respectively, what you can see and what you can do.
Cognitively, these are mapped to (so to speak) two cognitive processes - interpretation and action, which on a lower level some will define as 'pattern matching' and 'statistical coherence'.
On a modal window, the text is part of the content (interpretation), while buttons serve as interaction (action) elements.
Users start with the buttons.
Modals are often a barrier in the user's wish to complete a task. As such users often seek for action elements first.
Because of this, the common recommendation is to turn this:

download bmml source – Wireframes created with Balsamiq Mockups
Into that:

download bmml source
But notice that while in the first example the buttons are logical answers to the question, the buttons in the second example are not. To the question "are you sure?" one should be offered "yes" or "no" options. Similarly, in your modal, "Discard changes", "Save changes" and "Stay on page" are not answers to the question "Save changes?".

download bmml source
To the answers "keep customer" and "delete customer", one should ask:

download bmml source
But this could make little sense - the user has just clicked 'delete' so why is the user being asked what she would like to do? The modal is not reflecting its nature as a guard.
Playing blackjack
Consider I play blackjack in a friendly casino. Now my cards add up to 20, and the dealer asks me:
- "Hit or stand".
- "Hit", I reply
- "Are you sure?", asks the dealer.
There are two options here:
- I know what I'm doing and understand why the question is being asked.
- I have no idea what the rules of the game are.
Similarly, a user faced with a delete-guard modal may or may not have intended to delete something. So either the user just wants to progress, or they need to know why the warning is given (that is, what are the rules of the game). The thing is, that question don't do great in explaining these things. So in the following dialog:

download bmml source
importance is really bottom to top:
- Actions first
- Then reason
- Then question (arguably not needed at all).
So possibly, the ideal form is:

download bmml source