There's two major lines of thought on why Muslims fast.
One is that it's scientifically proven to be healthy, cleans the body and mind, and so on. There's plenty of internet resources supporting this, but I feel that much of it is flimsy because it's trying to find proof for a belief rather than believing in proof. One issue is that if it was suddenly proven that fasting is unhealthy (like in the book Freakonomics), the Muslims who claim that they're fasting because it's healthy will suddenly be in denial. They'd fast anyway, proving that they're not doing it for wordly reasons.
The other which I agree with more, is that it's a direct commandment. There are 5 pillars of Islam
that are followed by all
Muslims regardless of sect - Fasting during Ramadan is one of them. The Shia' follow a few more but also have mandatory fasting.
It's a prerequisite to fast in order to call yourself a Muslim. Someone who doesn't fast is, by strict definition, not a Muslim. Of course, plenty of Muslims don't fast, but they certainly keep it hidden.
Actually trying to explain why loyalty is sincere is much more difficult because of the cultural differences. Pretty much every other question about Muslims like "Why wear a hijab?", "Why not pork?" or "Why pray several times a week?" boils down to that same answer.
Islam is a religion of submission. A Muslim must only worship Allah and only ask for help from Allah
. Worship comes in the form of following Allah's commandments.
Why? This is a much deeper and more personal question. A lot of the Muslim world doesn't hold the same contempt for submission as the Western world does. Submission here is a form of humility. The humility to accept that there is a more superior power. There is the acceptance that everything comes from Allah. Wealth, health, happiness. Donate money in the cause of Allah, and you'll get back 300 times that money. If someone threatens to harm you, Allah keep you safe. If you die by doing something for the cause of Allah - by starving in the path of justice, getting assassinated for refusing a bribe - you are a martyr and get rewarded in the afterlife.
This kind of submission brings peace. There's no fear of death, starvation, poverty. A lot of devout Muslims are very independent adventurer types.. they'd go out and do what they wish, believing that Allah protects them, helping people without asking for any reward because they believe that Allah would reward them for their deeds in both cash and afterlife. The devout ones are incorruptible because they fear Allah's punishment more than the punishments and bribes of others. There's a higher purpose to everything in life and happiness comes from letting go. Things like fasting and giving away 2.5% of your wealth to charity is part of the process of humility and not trying to keep such a tight rein on life.
Of course, you won't find this written among many theologists and Islamic scholars because many scholars have their own personal agendas and totally miss the philosophy of the religion that one could find by looking back in its early history.