Why is certainty considered a condition for a prayer to be answered?

When asking Allah, one must be certain of the answer and have presence of heart, and think well of Allah; because Allah Almighty does not answer the supplication from a heedless, inattentive heart, as stated in the Hadith Qudsi: "I am as My servant thinks of Me," and His (the Most High) saying: "Call upon Me; I will respond to you," and "I respond to the invocation of the supplicant when he invokes Me."