No, its not a stupid question : )
Sawn (fasting) is one of the Pillars of Islam, A person who follows Islam, has to believe in the some "aqidah/Iman|
The six articles of belief are:
Belief in God (Allah), the one and only one worthy of all worship (tawhid).
Belief in the Angels (mala'ika).
Belief in the Books (kutub) sent by Allah(including the Quran, Bible and Tourah).
Belief in all the Messengers (rusul) sent by Allah (including Muhammad, Jesus, Moses, Abraham, Noah, and Adam)
Belief in the Day of Judgment (qiyama) and in the Resurrection (life after death).
Belief in Destiny (Fate) (qadar).
And then come the Pillars of Islam, they are
. Shahada
. Salat
. *Sawm*
. Zak?t
. Hajj
So, a Muslim has to fast no matter what.
Why do we fast ? :-
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)”
It's also one of the acts of worshiping Allah, we fast for him, we obtain from things that can harm others etcs,
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.”
http://islamdom.blogspot.com/2007/09/why-do-muslims-fast-during-ramadan.html
But if you're asking why do Muslims fast in the month of Ramadan (only, why not other month?) hmm that is something hard to answer, maybe because it is the month in which Allah revealed the Quran as a guidance for the whole of mankind. Well, wait I got a question to ask here:P
I recommend you to read :
http://peripateticwayfarer.wordpress.com/2010/08/08/ramadan-the-recession-during-which-the-satan-is-laid-off/:
Hope this is a satisfactory answer :)