Declining invitation

Take the following hypothetical example of declining an invitation?

Good day
Thank you for your invitation.
As much as I do appreciate your invitation
I regret being unable to attend at this time.
May God bless you in your endeavours.
Yours faithfully

Are there occassions that you\ve declined invitations and, if so, fo what reasons?
Are there yet other occassions that you were hoping for an invitation that never came?