Is it a Sin to alter or to add your preferences or beliefs in the way of Islam?

I know its a sin to add anything by our own wishes because Islam is been completed by Allah through Quran and Muhammad (p.b.u.h.) BUT my question is, who are we to change anything out of our wishes? http://answers.yahoo.com/question/index;_ylt=Au3Uex14rMS6IZzpAAHRcBPn7BR.;_ylv=3?qid=20080419234214AAnKRvI The... show more I know its a sin to add anything by our own wishes because Islam is been completed by Allah through Quran and Muhammad (p.b.u.h.)

BUT my question is, who are we to change anything out of our wishes?
http://answers.yahoo.com/question/index;...

The third option which the questioner has mentioned, is it wise to do so?

Why do we say Lailaha illallah Muhammdur Rasool Allah..??

We say this because Allah says in Quran:
"Say (O Muhammad): `O mankind, verily I am sent to you all as the Messenger of Allaah - to Whom belongs the Dominion of the heavens and the earth."' [Quran 7:158]

And : "Indeed in the Messenger of Allaah there is a good example to follow for him who hopes in (the meeting with) Allaah and the Last Day and remembers Allaah much." [Quran 33:21]

My question is clear. Make me understand s to why people bring changes?
And PLEASE plz plz, not bashing Shias, no not at all. I just want to know the REASON behind it.
Update: Drom57, Do u know the meaning of Ignorants? Do you know anything about islam? if no then please don't bother to comment.And the question I have asked is been directed to Muslims. Non-muslims cannot answer this question so please, Back off!
Update 2: Mohammad, m always here. Can u too please answer the question, m really curious to know the reason and what muslims think of it.
Update 3: Sina Can you give the references about this: Al Khumeini said that Muhammed Sala Allah Aleih Wa Salam (and all other prophets {his exact words}) has failed in completing the Islam and making justice and we have to wait for Al Mahdi....????
Update 4: Sina ....hmmm No english?
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