Praise be to Allaah.
Firstly:
We Muslims fast the month of Ramadaan because Allaah has commanded us to do so. Allaah says (interpretation of the meaning):
“ O you who believe! Observing As-Sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqoon (pious)”
[al-Baqarah 2:183]
So we worship Allaah by doing this act of worship which is beloved to Allaah and which He has enjoined upon us.
The believers hasten to obey the commands of Allaah and His Messenger (peace and blessings of Allaah be upon him), in obedience to His words (interpretation of the meaning):
“The only saying of the faithful believers, when they are called to Allaah (His Words, the Qur’aan) and His Messenger, to judge between them, is that they say: ‘We hear and we obey.’ And such are the successful (who will live forever in Paradise)”
[al-Noor 24:51]
“It is not for a believer, man or woman, when Allaah and His Messenger have decreed a matter that they should have any option in their decision. And whoever disobeys Allaah and His Messenger, he has indeed strayed into a plain error”
[al-Ahzaab 33:36]
Secondly:
It is by His wisdom that Allaah has prescribed a variety of acts of worship, so as to test people with regard to how they will obey all these commands. Will they only choose to do that which suits them, or will they do that which pleases Allaah? If we think about the five acts of worship: testimony of faith, prayer, zakaah, fasting and pilgrimage, we will see that some of them are purely physical, some are purely financial, some are both, so that the miser will become distinct from the generous. For some people it may be easy for them to pray one thousand rak’ahs but not to give a single dirham; for others it may be easy to give a thousand dirhams but not to pray a single rak’ahs. So Islam came to prescribe a variety of acts of worship so as to determine who will follow in obedience to the command of Allaah and who will follow only that which suits him.
Prayer, for example, is a purely physical action, but its prerequisites require some expenditure, such as the water for wudoo’, and clothes to cover the ‘awrah. These are not part of the prayer but they are its prerequisites.
Zakaah is purely financial, but physical actions are required to fulfil this duty such as calculating one's wealth and transferring the zakaah to the poor and needy. These are not part of zakaah but they are its prerequisites.
Hajj involves spending wealth and physical action, except for the people of Makkah who may not need money, but the yare very few compared with those who live in Makkah.
Commands are of two types: commands to refrain from things that man is inclined towards, and commands to spend that are precious.
Refraining from things that are loved includes fasting, and expenditure of things that are loved includes zakaah. Wealth is something that is loved and no one spends the wealth that he loves except for something that is loved even more.
The same applies to refraining from things that are loved, for a person may like to spend a thousand dirhams, but not fast a single day, or vice versa.
Shaykh Ibn ‘Uthaymeen, al-Sharh al-Mumti’, 6/190.
Thirdly:
There is another great reason why fasting is prescribed, which has been discussed in part in the answer to question no. 26862.
Shaykh Ibn ‘Uthaymeen was asked about the reason why fasting was enjoined?
He replied:
If we read the words of Allaah (interpretation of the meaning):
“ O you who believe! Observing As-Sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqoon (pious)”
[al-Baqarah 2:183]
we will know the reason why fasting was prescribed, which is taqwa (piety) and submission to Allaah. Taqwa means giving up haraam things, and in general terms includes both doing what is commanded and abstaining from what is forbidden. The Prophet (peace and blessings of Allaah be upon him) said: “Whoever does not give up false speech and acting upon it and offensive speech and behaviour, Allaah has no need of his giving up his food and drink.” Narrated by al-Bukhaari, 6057.
Based on this, it is important for the one who is fasting to carry out religious duties and avoid haraam things in word and deed. So he should not backbite about people, tell lies, or spread malicious gossip among them, or engage in haraam transactions, and he should avoid all haraam things. If a person does that for a whole month, the rest of the year will go well, but unfortunately in the case of many of those who fast, there is no difference between a day when they fast and a day when they do not; they behave as they usually do, neglecting obligatory duties and doing forbidden things. You do not see the dignity that is to be expected of the fasting person. These actions do not invalidate their fast but they do detract from its reward and may cancel out the reward altogether.
Fataawa Arkaan al-Islam, p. 451.
[ RAMADAN ] WHY FAST ? - NOUMAN ALI KHAN
THE HISTORY OF FASTING - DR ZAKIR NAIK