Assalamu Alaykum,
You said: " if you are extremist tell the reason if not why?"
Firstly, I think you should have asked your question in a better way. I really dont know what the point of this thread is. Muslims are not extreme, no we hate extremism. It has nothing to do with Islam. If there is anyone who says they are extreme, they should either LEARN Islam and follow it properly, Or they can get lost for they are hated in the eyes of Allah, the most high.
Here is why>>
Islam is complete and perfect:
This day have I perfected your religion for you, completed My favour upon you, and have chosen for you Islam as your religion. - [Quran 05:03]
Its always the middle way, A Muslim should never be extreme nor should he forget his Lord and the reason of his existance, i.e. to worship Allah the most High.
Allah the Most high and the Prophet
instructed us on how to follow Islam.
1. To understand Islam properly>>
We Follow the Quran and we also follow the Hadith:
And whatever the Messenger gives you, take it, and whatever he forbids you, leave it. And fear Allah: truly Allah is severe in punishment. " [Quran 59:7]
Say: "Obey Allah and obey the Messenger, but if you turn away, he (Messenger Muhammad saaws) is only responsible for the duty placed on him (i.e. to convey Allah's Message) and you for that placed on you. If you obey him, you shall be on the right guidance. The Messenger's duty is only to convey (the message) in a clear way (i.e. to preach in a plain way)." (Quran 24:54):
Allaah's Messenger
gave us an admonition which caused the eyes to shed tears and the hearts to fear, so we said, "O Messenger of Allaah, this is as if it were a farewell sermon, so with what do you counsel us?" So he said: "I have left you upon clear proof , its night is like its day, no one deviates from it except one who is destroyed, and whoever lives long from amongst you will see great controversy. So stick to what you know from my Sunnah and the Sunnah of the orthodox, rightly-guided caliphs - cling to that with your molar teeth, and stick to obedience even if it is to an Abyssinian slave, since the believer is like a submissive camel, wherever he is led, he follows" [Ahmad (4/126), Ibn Maajah (no. 43), al-Haakim (1/96) and others - Hasan]
The Prophet
also said:
"Hold on to my Sunnah and the Sunnah of the Rightly Guided Caliphs after me. Hold on to it firmly. And beware of heretical innovations (Bid'ah) because each heretical innovation (Bid'ah) is a falsehood and each false- hood is a deviation from the right path." (Reported by Ahmad Ibn Hanbal Abu Dawud, at-Tirmidhi who said it is a good and sound hadith, and Ibn Majah)
Following the right way
The Correct Creed
Islam, the Religion of Ease and Moderation
Allah, the most Glorified and Most High, is the sole creator of ALL mankind and therefore knows his nature more intricately than any of mankind.
Allah, has therefore chosen for us a religion best suited to the nature of mankind, a religion that goes neither to the Extremes of hardship nor of laxity, but instead provides a middle path; in other words, a religion of ease.
Allah, most high stated in the Quran:
"Allah intends for you ease, and does not want to make things difficult for you" [2:185];
"Allah does not want to place you in difficulty" [5:6].
Such easiness is well explained in the hadeeth reported by Abu Hurairah, radiya Allahu 'anhu, that the Prophet, salla Allaahu 'alaihe wasallam, said, "Religion is easy..." [Bukhari], he also said; "The best of your religion, is the easiest." [Ahmad]
The easiness of this religion was put into practise by the best of humanity, the one who came to deliver the message, as Allah, subhanahu wata'aala, said;
"Verily there has come unto you a Messenger from amongst yourselves, it grieves him that you should suffer any difficulty, he is anxious for you, for the believers he is full of pity and merciful" [10:128]
Therefore a middle way is always taken. Patience is one of the greatest virtues a Muslim can have.
"And verily, whosoever shows patience and forgives, that would truly be from the things recommended by Allah.'' (42:43)
The Messenger of Allah 
saw said,"Allah shows compassion only to those among His slaves who are compassionate".
[Al-Bukhari and Muslim].
The Prophet Spoke against extremism.
The Prophet (peace and blessings be upon him) said, “This faith is easy and whosoever wrestle with it (i.e., takes it to extremes and overburden himself with difficult duties) will end up defeated. So, follow the path of rectitude and moderation.” (Al-Bukhari)
The Prophet eace and Blessings be upon him) also said:
‘Beware of extremism in matters of religion. For truly, those before you were destroyed on account of (their) religious extremism.’ (Ahmed & Nisai)
Abu Hurairah (May Allah be pleased with him) reported:The Messenger of Allah saw said, "The strong man is not one who is good at wrestling, but the strong man is one who controls himself in a fit of rage.''
[Al-Bukhari and Muslim].
and Allah the most High says in the Quran:
"Show forgiveness, enjoin what is good, and turn away from the foolish." (Al-A`raaf 7:199)
