That is because Muslims believe that Allah sent the Islamic religion as the final religion, that it is obligatory for all human beings who hear of it to believe in. Your friend wants to save you from the hellfire. Also, besides the threat of the hellfire, Muslims just love Islam. We want others to experience the joy that we feel from this religion insha'Allah(God willing)
There are five verses of the quranQur'an and one hadith (saying of the Prophet(pbuh PBUH) that make it clear why we were created and how we must worship.
It was reported from Abu Hurayra that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “By the One in whose hand is the soul of Muhammad, not one of this nation, Jew or Christian, will hear of me and will die without having believed in that with which I have been sent, except that he will be one of the dwellers of Hell fire.”
It was reported from Abu Hurayra that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: "By the One in whose hand is the soul of Muhammad, not one of this nation, Jew or Christian, will hear of me and will die without having believed in that with which I have been sent, except that he will be one of the dwellers of Hell fire."
And I did not create the jinn and mankind except to worship Me. (Quran 51:56)
And whoever desires other than Islam as religion - never will it be accepted from him, and he, in the Hereafter, will be among the losers. (Quran 3:85)
This day have I perfected your religion for you, completed My favor upon you, and have chosen for you Islam as your way of life. (Quran 5:3)
And We send not the Messengers but as givers of glad tidings and as warmers. So whosoever believes(in Islam) and does righteous good deeds, upon such shall come no fear, nor shall they grieve. (Quran 6:48-49)
Allah forgiveth not that partners should be set up with Him(polytheism); but He forgiveth anything else, to whom He pleaseth; to set up partners with Allah is to devise a sin most heinous indeed. (Quran 4:48)