I live in England, and I often get asked by
many non-muslims, why do muslims fast? I
know that I should know this answer, but I
do not know what to say exactly. What
should I give as an answer?.

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.
Jihad for the sake of Allaah may require both
money and physical effort. A person may
spend money for the sake of Allaah and not
fight, or he may go and fight but not spend
money.
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 and drink.” Narrated by
al-Bukhaari, 607
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.


Source: http://islamqa.info/en/38064