- If you get cancer, the only help you'll get is from a doctor.
- If you get HIV, the only help you'll get is from a doctor.
- If you get the flu, the only help you'll get is from medicine to help with symptoms.
The above thinking goes against Islamic beliefs, tantamount to shirk. Allah may be the "best of protectors", but He is also the one who gives life and causes death. Doctors can only do so much.
The ultimate meaning of Islam is "submission", and dua is an expression of that. When making dua, we are submitting ourselves to His will, and recognizing that anything we accomplish, anything that happens to us, good or bad, is subject to that will. He may answer your dua immediately, or later, or replace it with something better, or even reject it completely.
Abandoning dua because He doesn't respond immediately or in the manner you expect is a weakness of faith.
In other words, Allah is the almighty creator of the universe: He is not a genie in a bottle that you get to rub and make a wish. Dua helps us remember that. Meaning Dua makes us remember that Allah is not our slave, instead we are the slaves of Allah. The act of asking from Allah (through prayer) is the testification that he is our Master and in it is our submission and surrender.
If you expect Allah to grant every wish, then you are expecting Allah to be your slave, and you to be dependent on a slave (not Master).